Green, Blue and Grey Hydrogen: A Complete Colour-Coded Guide

Hydrogen is one of the most discussed molecules in the global decarbonisation conversation. It appears in government energy strategies, industrial roadmaps, infrastructure investment plans, and corporate net zero commitments with a frequency that has grown dramatically over the past five years. Alongside that growth in attention has come a parallel growth in ambiguity, because hydrogen, unlike electricity or natural gas, does not arrive with an automatically visible carbon footprint. Two facilities consuming the same volume of hydrogen may be generating dramatically different levels of greenhouse gas emissions, depending entirely on how that hydrogen was produced.

This is the purpose of the colour classification system. It exists to distinguish between hydrogen production pathways on the basis of their carbon intensity, providing a shorthand that allows policymakers, investors, industrial buyers, and sustainability professionals to communicate meaningfully about hydrogen without having to specify the full production pathway every time. Understanding what each colour means, where the boundaries between them lie, and what the limitations of the classification system are, is now a baseline requirement for anyone engaged seriously with industrial decarbonisation strategy.

Grey Hydrogen - The Dominant but Dirty Default

Grey hydrogen is the hydrogen the world currently runs on. It accounts for more than 95% of all hydrogen produced globally, and it is the foundation of a hydrogen economy that, despite all the attention paid to green alternatives, remains overwhelmingly dependent on fossil fuels.

Grey hydrogen is produced through a process called Steam Methane Reforming, commonly known as SMR. In this process, natural gas, which is primarily methane, is reacted with steam at high temperatures, typically between 700 and 1000 degrees Celsius, in the presence of a catalyst. The reaction produces a mixture of hydrogen and carbon monoxide known as synthesis gas or syngas. A secondary reaction, the water-gas shift reaction, then converts the carbon monoxide and additional steam into carbon dioxide and more hydrogen. The CO2 produced in this process is released directly into the atmosphere without capture.

The carbon intensity of grey hydrogen production is substantial. Every tonne of hydrogen produced via SMR emits between 9 and 12 tonnes of CO2, depending on the efficiency of the plant, the carbon intensity of the natural gas feedstock, and the energy source used to provide the process heat. At current global hydrogen production volumes of approximately 94 million tonnes per year, grey hydrogen production alone is responsible for roughly 830 million tonnes of CO2 emissions annually, equivalent to the combined annual emissions of Germany and the United Kingdom.

The economic case for grey hydrogen has historically been compelling. Production costs sit in the range of one to two dollars per kilogram, making it the cheapest hydrogen production pathway available at scale. This cost advantage has embedded grey hydrogen deeply into the industrial economy, where it serves as a critical feedstock in ammonia production for fertilisers, as a process input in oil refining for desulphurisation and hydrocracking, as a raw material in the production of methanol and other chemicals, and as a direct reductant in certain steel production processes.

The fundamental problem with grey hydrogen from a climate perspective is equally straightforward. Using grey hydrogen does not decarbonise anything. It simply relocates the point of emission from the end use facility to the hydrogen production facility. A steelmaker that switches from coking coal to grey hydrogen as a reducing agent is not reducing its total value chain emissions in any meaningful way. It is rearranging where those emissions appear in the accounting framework.

Any company that refers to a hydrogen strategy without specifying the production pathway, or that claims emissions benefits from hydrogen use without disclosing the source of that hydrogen, should be asked a simple question: what colour is it?

Blue Hydrogen - The Transitional Compromise

Blue hydrogen begins with the same production process as grey hydrogen. It uses the same Steam Methane Reforming technology, the same natural gas feedstock, and produces the same hydrogen and CO2 outputs. What distinguishes blue hydrogen from grey is the addition of Carbon Capture and Storage technology, commonly referred to as CCS, which intercepts the CO2 produced during the reforming process before it reaches the atmosphere and transports it to a geological storage site where it is permanently sequestered underground.

The theoretical appeal of blue hydrogen is significant. If the CCS system attached to an SMR facility can capture the majority of the CO2 produced and store it permanently, the net emissions from hydrogen production can be reduced dramatically relative to grey hydrogen, while using the same well-understood production technology and the existing natural gas infrastructure. This makes blue hydrogen attractive as a near-term decarbonisation pathway for sectors that need hydrogen at scale today and cannot wait for green hydrogen supply chains to develop at the required cost and volume.

The practical reality of blue hydrogen is more complicated, and the complications matter significantly for how it is assessed within ESG and net zero frameworks.

The first complication is capture rate. Current commercial CCS systems attached to SMR facilities typically achieve CO2 capture rates of between 85% and 95% of the CO2 produced during the reforming process itself. This means that even under optimistic assumptions, between 5% and 15% of the reforming CO2 escapes into the atmosphere. For a production pathway that was already emitting 9 to 12 tonnes of CO2 per tonne of hydrogen, a 90% capture rate still results in approximately 1 tonne of CO2 per tonne of hydrogen reaching the atmosphere from the production process alone.

The second complication is methane leakage. The SMR process requires a continuous supply of natural gas, and natural gas supply chains leak methane at various points from extraction and processing through pipeline transmission and distribution. Methane is a potent greenhouse gas with a global warming potential approximately 80 times that of CO2 over a 20-year time horizon. Even relatively small methane leakage rates across the natural gas supply chain can substantially erode the climate benefit of the CCS system attached to the SMR facility. Studies have found that for blue hydrogen to deliver a net climate benefit relative to direct natural gas combustion, methane leakage rates across the supply chain must be kept below approximately 1.5%, a threshold that is not consistently achieved across all gas supply chains currently in operation.

The third complication is the permanence and verifiability of geological CO2 storage. CCS technology has been demonstrated at commercial scale in a small number of projects globally, but the long-term monitoring, reporting, and verification of stored CO2 remains a significant challenge. The climate benefit of blue hydrogen is only as real as the permanence of the storage behind it, and the regulatory and verification frameworks for long-term geological storage are still maturing.

None of these complications mean that blue hydrogen has no role in the energy transition. For heavy industrial sectors that require large volumes of low-carbon hydrogen in the near term and that are located in geographies with access to suitable geological storage formations and low-leakage gas supply chains, blue hydrogen can serve as a credible and meaningful transitional pathway. The critical point is that it must be evaluated rigorously rather than assumed to be clean, and it must be accompanied by a clear roadmap toward green hydrogen as costs and infrastructure develop.

Production costs for blue hydrogen currently sit in the range of one dollar fifty to three dollars per kilogram, depending on the cost of natural gas, the capital and operating costs of the CCS system, and the availability of transport and storage infrastructure. This places blue hydrogen at a cost premium to grey but significantly below green hydrogen at current production costs.

Green Hydrogen - The Real Prize

Green hydrogen is produced through the electrolysis of water using electricity from renewable sources. An electrolyser passes an electric current through water, causing it to split into its constituent elements: hydrogen at the cathode and oxygen at the anode. The hydrogen is then collected, compressed, and either used directly or stored for later use. When the electricity powering the electrolyser comes from wind, solar, hydroelectric, or other genuinely renewable and additional sources, the entire production process generates zero operational CO2 emissions. The only outputs are hydrogen and oxygen.

This fundamental characteristic, the absence of carbon in both the feedstock and the process, makes green hydrogen the only hydrogen production pathway that is fully consistent with net zero commitments and with the emissions trajectories required by the Paris Agreement. It is not a transitional technology with residual emissions that need to be managed and monitored. It is, at its core, a clean production process that produces a clean energy carrier.

The primary barrier to green hydrogen at present is cost. Current production costs sit in the range of three to six dollars per kilogram, with significant variation depending on the cost of renewable electricity, the scale and efficiency of the electrolyser, the capacity factor at which the electrolyser operates, and the cost of capital in the relevant market. In regions with excellent renewable resources, such as the Middle East, northern Africa, Chile, and parts of Australia, the combination of very low-cost renewable electricity and high capacity factors is already pushing green hydrogen production costs toward the lower end of this range, and in some cases below it.

The cost trajectory for green hydrogen is moving in the right direction, and the pace of movement is accelerating. Electrolyser manufacturing capacity is scaling rapidly, driven by policy incentives in the European Union, the United States, India, Japan, and other major economies. The learning rate for electrolyser technology, meaning the cost reduction achieved for each doubling of cumulative installed capacity, is comparable to that observed in solar photovoltaic manufacturing over the past two decades. The International Renewable Energy Agency projects that green hydrogen production costs could fall to between one dollar fifty and two dollars fifty per kilogram by 2030 in regions with strong renewable resources, placing it at or below the unsubsidised cost of blue hydrogen.

The electrolysis technology itself exists in several variants that are at different stages of commercial maturity. Alkaline electrolysis is the most established technology, with decades of industrial deployment history, relatively low capital costs, and a well-understood operational profile. Proton Exchange Membrane electrolysis offers higher efficiency and greater flexibility in responding to variable renewable power inputs but currently carries higher capital costs. Solid Oxide electrolysis operates at high temperatures and offers very high electrical efficiency but is still at an earlier stage of commercial development. Each technology has applications where it is better suited than the others, and the competitive landscape is evolving rapidly.

For green hydrogen to deliver its full climate benefit, it must be produced using electricity that is genuinely renewable and genuinely additional. This requirement, known as the additionality principle, is central to the emerging regulatory frameworks for green hydrogen in the European Union and other jurisdictions. Hydrogen produced using grid electricity that is not backed by additional renewable generation may have a carbon intensity that varies with the carbon intensity of the grid at any given moment, and in grids with significant fossil fuel generation, that carbon intensity may be higher than assumed. The requirement for temporal correlation, meaning that the renewable electricity used to produce hydrogen must be generated at the same time as it is consumed, is becoming an increasingly important element of green hydrogen certification frameworks, particularly in Europe.

The certification and verification of green hydrogen is handled through Guarantees of Origin in Europe and equivalent instruments in other jurisdictions. These certificates provide a chain-of-custody record that links a specific volume of hydrogen to a specific renewable electricity source, enabling buyers to make credible claims about the carbon intensity of the hydrogen they consume.

The Wider Colour Spectrum

While grey, blue, and green represent the three production pathways that dominate current policy and investment discussions, the hydrogen colour spectrum extends further, and several additional colours are worth understanding as the market evolves and new production technologies develop.

Turquoise hydrogen is produced through a process called methane pyrolysis, in which natural gas is thermally decomposed into hydrogen and solid carbon rather than CO2. Because the carbon output is a solid material rather than a gas, it can potentially be sequestered or used as an industrial input, such as in tyre manufacturing or electrode production, without being released into the atmosphere. Methane pyrolysis still requires natural gas as a feedstock, which means it carries upstream methane leakage risks similar to blue hydrogen, but it avoids the need for geological CO2 storage infrastructure. The technology is at an early commercial stage, with several pilot and demonstration projects underway but no large-scale commercial deployment yet established.

Yellow hydrogen refers to hydrogen produced through electrolysis powered specifically by solar energy. It is functionally equivalent to green hydrogen in terms of its operational carbon intensity, but the colour distinction is used in some contexts to highlight the specific renewable source, which has implications for the temporal availability of production, given the intermittent nature of solar generation.

Pink hydrogen is produced through electrolysis powered by nuclear energy. Nuclear electricity has near-zero operational carbon emissions and, unlike wind and solar, can operate at high and consistent capacity factors independent of weather conditions. This makes nuclear-powered electrolysis an attractive option for producing large volumes of hydrogen at high consistency, which is an important operational characteristic for industrial users with steady hydrogen demand. Several countries with significant nuclear capacity, including France, South Korea, and Canada, are actively developing pink hydrogen programmes.

White hydrogen refers to naturally occurring geological hydrogen found in underground deposits. Hydrogen has been discovered in commercially interesting concentrations in several locations globally, including Mali, where a village has been running on naturally occurring hydrogen for decades, and in geological formations across the United States, Australia, and the Iberian Peninsula. While the scale of geological hydrogen resources remains poorly characterised and the commercial extraction technology is undeveloped, the potential for a hydrogen source with minimal production emissions and very low cost is attracting growing scientific and investment interest.

Hydrogen Colour and ESG Strategy

The hydrogen colour classification has direct and material implications for how companies account for and report their emissions under the GHG Protocol, CSRD, and other established frameworks.

Grey hydrogen consumed in industrial operations contributes directly to a company's carbon footprint. If the hydrogen is produced on-site, those emissions fall within Scope 1. If it is purchased from an external supplier, the associated emissions fall within Scope 3 Category 1, purchased goods and services. In either case, the emissions are real, they are substantial, and they must be reflected in the company's emissions inventory.

For companies in steel, cement, chemicals, refining, and other heavy industrial sectors where hydrogen is a significant process input, the transition from grey to green hydrogen represents one of the highest-impact decarbonisation levers available. It is also one of the most capital and infrastructure intensive. Executing a grey-to-green hydrogen transition requires not just a procurement decision but a comprehensive strategy that addresses electrolyser sourcing or green hydrogen supply contracting, renewable electricity procurement with appropriate additionality guarantees, infrastructure adaptation at the point of use, and the financial modelling of a transition that involves significantly higher input costs in the near term in exchange for long-term carbon and regulatory risk reduction.

Blue hydrogen can serve a legitimate role as an interim solution for companies that need to make near-term emissions reductions while green hydrogen supply chains develop at the required scale and cost. However, companies relying on blue hydrogen in their decarbonisation strategies must be prepared to substantiate the emissions claims they make on its basis. This requires verified CCS performance data showing actual capture rates rather than design specifications, independent audits of upstream methane leakage rates across the gas supply chain, and transparent disclosure of the residual emissions that remain after CCS. Regulatory frameworks and third-party verifiers are becoming increasingly sophisticated in scrutinising these claims, and blue hydrogen assertions that cannot be backed by rigorous data are likely to attract growing challenge from investors, regulators, and civil society.

The direction of the hydrogen economy is not ambiguous. The policy capital, the investment capital, and the technology development trajectories are all aligned behind green hydrogen as the long-term destination. The scale and pace of that transition will vary by sector, geography, and application, and blue hydrogen will play a role in bridging the gap in some contexts. But for companies building net zero strategies that need to remain credible through 2030, 2040, and 2050, the question is not whether green hydrogen is the answer. The question is how to get there in a way that is technically grounded, financially viable, and transparent enough to withstand the scrutiny that ESG commitments of this significance will inevitably attract.

Grey is the problem. Blue is the bridge. Green is the destination. Understanding the difference is no longer optional.

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