Introduction
Types (Colours) of Hydrogen.
Hydrogen is colourless. The colours commonly attached to it describe how it was produced—not the appearance or quality of the gas.
The colour system is a convenient shorthand for discussing production pathways and their likely environmental effects. Green usually means hydrogen made by electrolysis using renewable electricity; grey usually means hydrogen made from natural gas without carbon capture; and blue generally means fossil-based hydrogen with carbon capture and storage.
Other labels describe coal gasification, nuclear-powered electrolysis, methane pyrolysis, naturally occurring geological hydrogen and emerging methods that stimulate hydrogen underground.
The system is useful for a quick introduction, but it is not an internationally agreed technical standard. Definitions can differ between publications, and the same colour can conceal large differences in actual greenhouse-gas emissions.
The most reliable comparison is therefore the measured lifecycle emissions of each kilogram of hydrogen, together with its production method, energy source, feedstock and carbon-management performance.
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The Colour Classification at a Glance
The Colour Classification at a Glance
The label normally identifies the feedstock, production process and source of energy used to make the hydrogen.
| Common Hydrogen Colour Classification | |||
|---|---|---|---|
| Colour | Feedstock or source | Production pathway | Main emissions consideration |
| Green | Water and renewable electricity | Electrolysis | Electricity source and full supply-chain emissions |
| Pink or purple | Water and nuclear energy | Electrolysis, sometimes with nuclear heat | Lifecycle emissions and treatment of nuclear generation |
| Yellow | Water and electricity | Electrolysis using solar or mixed grid electricity, depending on the source | The term is inconsistent; the actual electricity mix matters |
| Grey | Natural gas | Steam methane reforming without carbon capture | Process carbon dioxide and upstream methane emissions |
| Blue | Natural gas or coal | Reforming or gasification with carbon capture, utilisation and storage | Capture rate, residual emissions and upstream fuel emissions |
| Brown or black | Lignite or black coal | Coal gasification without carbon capture | High direct greenhouse-gas emissions |
| Turquoise | Methane | Pyrolysis producing hydrogen and solid carbon | Heat source, methane leakage and permanent use or storage of the carbon |
| White or gold | Naturally occurring geological hydrogen | Extraction from underground accumulations | Exploration, leakage, purity, processing and field operations |
| Orange | Water, carbon dioxide and reactive rock | Stimulated geological production | Still emerging; field-scale performance and lifecycle effects remain uncertain |
This table describes common usage rather than fixed legal definitions. Individual projects should state their actual production pathway and provide an emissions calculation rather than relying on a colour alone.
Green Hydrogen
Green Hydrogen
Green hydrogen is produced by splitting water in an electrolyser powered by renewable electricity.
Wind, solar, hydroelectric, geothermal and other renewable generation can supply the electricity. The electrolyser separates water into hydrogen and oxygen, so the production equipment does not release carbon dioxide from a fossil feedstock.
The environmental result still depends on the complete system. Grid electricity can contain fossil generation; building equipment and renewable generation has embodied emissions; water must be treated; and compression, storage and delivery consume additional energy.
Claims should therefore explain whether the renewable electricity is supplied directly, matched through a contract or drawn from the grid, and how the timing and emissions of that electricity are counted.
New Zealand's high proportion of renewable electricity gives green hydrogen a strong potential role, particularly where direct electrification is difficult or where onsite production provides useful control over fuel supply.
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Pink, Purple and Yellow Hydrogen
Pink, Purple and Yellow Hydrogen
These labels describe hydrogen made from water using particular sources of electricity or heat, but their meanings are less consistent.
Pink or Purple Hydrogen
Pink and purple commonly refer to hydrogen produced by electrolysis using nuclear electricity. Some descriptions also include the use of nuclear heat in high-temperature electrolysis. The process can have low operational greenhouse-gas emissions, although the classification depends on how nuclear generation, construction, fuel processing and waste are assessed.
Yellow Hydrogen
Yellow is used inconsistently. Some sources apply it specifically to solar-powered electrolysis, while others use it for electrolysis supplied by a mixed electricity grid. Those two pathways can have very different emissions.
A project described as yellow hydrogen should therefore state its electricity source and measured emissions rather than assuming that the colour communicates enough information.
Grey, Blue, Brown and Black Hydrogen
Grey, Blue, Brown and Black Hydrogen
These established pathways make hydrogen from fossil fuels and differ mainly in the feedstock and treatment of the resulting carbon dioxide.
Grey Hydrogen
Grey hydrogen is usually produced from natural gas through steam methane reforming. The process separates hydrogen from methane and releases the resulting carbon dioxide to the atmosphere. Upstream natural-gas production and transport can add further emissions through energy use and methane leakage.
This has historically been the main pathway for hydrogen used in New Zealand's fertiliser, methanol and refining industries.
Blue Hydrogen
Blue hydrogen uses a fossil-fuel production pathway but captures and stores some of the carbon dioxide. The label alone does not specify the capture rate, the permanence of storage or the upstream methane emissions.
Two blue-hydrogen plants can consequently have very different lifecycle emissions. Credible assessment requires the amount of carbon captured across the whole plant, residual combustion emissions, electricity and heat inputs, methane leakage and the long-term performance of carbon storage.
Brown and Black Hydrogen
Brown hydrogen is generally made by gasifying lignite, while black hydrogen is made from higher-grade black coal. Without carbon capture these pathways are highly emissions-intensive. If carbon capture is added, some publications may instead place the product within the broad blue category.
Turquoise Hydrogen
Turquoise Hydrogen
Turquoise hydrogen is produced by heating methane without oxygen so that it separates into hydrogen and solid carbon.
This process is called methane pyrolysis. Unlike steam methane reforming, the principal carbon product is solid rather than a stream of carbon dioxide.
Its climate performance depends on the source of the process heat, upstream methane leakage and what happens to the solid carbon. If the heat is low-emissions and the carbon remains securely stored or is incorporated into a durable product, the pathway may have lower emissions than conventional grey hydrogen.
If fossil energy supplies the heat, methane leaks from the supply chain or the solid carbon is later oxidised, that advantage is reduced. Commercial maturity and the scale of markets capable of permanently using the carbon also matter.
White, Gold and Orange Hydrogen
White, Gold and Orange Hydrogen
These labels refer to hydrogen found or generated in geological formations rather than manufactured in a conventional surface plant.
White or Gold Hydrogen
Natural hydrogen is generated by geological and chemical processes underground. It is often called white hydrogen and is also described as gold hydrogen. Exploration aims to find accumulations that can be produced through wells.
The resource could avoid much of the electricity or fossil feedstock used in manufactured hydrogen, but commercial performance remains unproven at scale. Flow rate, purity, replenishment, processing, drilling impacts, leakage and distance from customers will determine its actual value.
Orange Hydrogen
Orange hydrogen describes an emerging approach in which water—and in some proposals carbon dioxide—is injected into suitable reactive rock to stimulate hydrogen-producing reactions underground. The intended process may produce hydrogen while retaining carbon in mineral form.
New Zealand is considering how natural and stimulated geological hydrogen could fit within the minerals and petroleum regulatory system. The potential scale, repeatability and environmental effects still require field evidence.
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Why Emissions Intensity Matters More Than Colour
Why Emissions Intensity Matters More Than Colour
A colour identifies a pathway, but it does not prove the environmental performance of a particular kilogram of hydrogen.
Green hydrogen made with genuinely renewable electricity can have low lifecycle emissions, while electrolysis using a fossil-heavy grid can be highly emissions-intensive. Blue hydrogen can range from comparatively low emissions to much higher emissions depending on carbon-capture performance and methane leakage.
The International Energy Agency does not use colour terminology in its analysis because the environmental effects of each pathway can vary greatly by energy source, region and technology. It recommends definitions based on measured greenhouse-gas emissions intensity.
A useful project comparison should identify:
- The feedstock and production technology;
- The source and timing of electricity and heat;
- Direct process emissions;
- Upstream fuel and methane emissions;
- The carbon-capture rate and storage outcome, where applicable;
- Compression, processing, storage and delivery energy; and
- The resulting kilograms of carbon-dioxide equivalent per kilogram of hydrogen.
The colour system remains a useful first question. Verified lifecycle emissions provide the answer needed for investment, policy and credible environmental claims.