Tech Q of the Week: The Hydrogen Color Scale

Information provided by Technical Response Team (January 2022)

*Image from https://www.rff.org/publications/reports/decarbonizing-hydrogen-us-power-and-industrial-sectors/

What are general definitions of green, blue, gray, and brown hydrogen?

Currently, we are not aware of industry-wide definitions of blue, green, gray, and brown hydrogen in the United States. That said, we have provided some definitions below that are sourced from state legislation, industry associations, and international organizations. Note that we can’t verify the accuracy of non-government resources. In general, each color is intended to represent a hydrogen production method and the source of production, such as renewables or fossil fuels. For example, green hydrogen represents a relatively low-polluting production method through renewables and brown hydrogen represents a higher-polluting production method through coal.


Before reviewing the “color spectrum” of hydrogen production, it might be helpful to first review descriptions of hydrogen production methods by visiting the Alternative Fuels Data Center Hydrogen Production and Distribution page (Source). Additionally, we suggest reviewing the U.S. Department of Energy’s (DOES) Hydrogen Production page (Source) for general information about the production process and DOE’s Natural Gas Reforming page (Source) for information about steam-methane reformation (SMR). It’s relevant to note that 95% of the hydrogen produced in the United States is made by natural gas reforming in large central plants.

First, as an example, the State of California Public Utilities Code includes a definition of green hydrogen

  • 400.2. For the purposes of this article, “green electrolytic hydrogen” means hydrogen gas produced through electrolysis and does not include hydrogen gas manufactured using steam reforming or any other conversion technology that produces hydrogen from a fossil fuel feedstock.” (source)

The educational organization, the Green Hydrogen Coalition, also offers a definition for green hydrogen and its production methods (Source):

  • Green hydrogen is defined as hydrogen created from renewable energy sources such as solar, wind, hydro power, biomass, biogas, or municipal waste.
  • Green hydrogen can be generated from renewable electricity such as solar or wind power by electrolysis, from biogas by steam reforming, or from biomass through thermal conversion.”

The Hydrogen Council provides definitions for green and gray hydrogen in their report, Path to Hydrogen Competitiveness (Source):

  • “Most hydrogen today is produced from fossil fuels and emits carbon (grey hydrogen). There are numerous options for producing low-carbon and renewable hydrogen. This report focuses on the two main options: reforming natural gas or coal and capturing the emitted carbon, and electrolysis using low-carbon power as an input. Biomass gasification is another promising source of low-carbon hydrogen production; however, it does not currently contribute a meaningfully large share of global supply. Two main technologies can produce hydrogen from electrolysis in combination with renewable electricity: proton-exchange membrane (PEM) and alkaline. Alkaline is currently the most mature technology, which uses a saline solution to separate hydrogen from water molecules by applying electricity. PEM is slightly less mature and uses a solid membrane to separate the hydrogen from water molecules via an electric charge.”

RMI provides a definition of green, gray, and blue hydrogen (Source):

  • “There are four major sources for commercial production of hydrogen, three of which require fossil fuels: SMR, oxidation, and gasification. The fourth source is electrolysis, which separates water into its constituent elements (hydrogen and oxygen) using electricity. When that electricity is produced through renewable resources you can have zero carbon green hydrogen. This is the only non-fossil fuel means of hydrogen production. The SMR process, which emits CO2, requires substantial heat to chemically separate the hydrogen from the methane molecules. When the emissions of that process are not captured, it is referred to as grey hydrogen. When carbon capture and storage (or carbon capture, utilization, and storage) is attached to a facility, it is referred to as blue hydrogen. In addition to SMR, hydrogen can also be synthesized from oil via partial oxidation, or from coal via gasification.”

Wood Mackenzie also provides definitions for types of hydrogen in their Hydrogen Guide (Source):

  • “How is hydrogen produced? — The vast majority (99.6%) currently comes from hydrocarbons. Around 71% is grey hydrogen, produced via the reforming of natural gas to produce CO2 and hydrogen. Most of the rest is brown hydrogen, from coal via gasification.
  • A small portion is blue hydrogen, a lower-carbon alternative that pairs natural gas reforming with carbon capture and storage (CCS). But CCS isn’t yet widely commercial.

What’s green hydrogen?

  • Green hydrogen is produced from water by renewables-powered electrolysis. Its green credentials will make it critical for difficult-to-decarbonise industries like steel– but in 2020 it only constitutes 0.1% of global hydrogen production. The economics are a challenge.”


Lastly, a story from National Grid titled The Hydrogen Colour Spectrum (Source) provides definitions for multiple types of hydrogen including green, blue, gray, and brown. Please note, National Grid is an international energy company and while this article is written towards the European market it is our understanding that these definitions generally apply in the United States:

  • “What is green hydrogen?
    • In the kaleidoscope of hydrogen colours, green hydrogen is the one produced with no harmful greenhouse gas emissions. Green hydrogen is made by using clean electricity from surplus renewable energy sources, such as solar or wind power, to electrolyze water. Electrolysis use an electrochemical reaction to split water into its components of hydrogen and oxygen, emitting zero-carbon dioxide in the process.
    • Green hydrogen currently makes up a small percentage of the overall hydrogen, because production is expensive. Just as energy from wind power has reduced in price, green hydrogen will come down in price as it becomes more common.


  • What is blue hydrogen?
    • Blue hydrogen is produced mainly from natural gas, using a process called steam reforming, which brings together natural gas and heated water in the form of steam. The output is hydrogen – but also carbon dioxide as a by-product. That means CCS is essential to trap and store this carbon.
    • Blue hydrogen is sometimes described as “low-carbon hydrogen” as the steam reforming process doesn’t actually avoid the creation of greenhouse gases.


  • What is grey hydrogen?
    • Currently, this is the most common form of hydrogen production. Grey hydrogen is created from natural gas, or methane, using steam methane reformation but without capturing the greenhouse gases made in the process.


  • What are black and brown hydrogen?
    • Using black coal or lignite (brown coal) in the hydrogen-making process, these black and brown hydrogen are the absolute opposite of green hydrogen in the hydrogen spectrum and the most environmentally damaging.”

If you are interested in learning more or have questions, please reach out Utah Clean Cities


Other colors that may be included within the Hydrogen Production Color Scale: (Source)

  • Turquoise hydrogen can be extracted by using the thermal splitting of methane via methane pyrolysis. The process, though at the experimental stage, remove the carbon in a solid form instead of CO2 gas.
  • Purple hydrogen is made though using nuclear power and heat through combined chemo thermal electrolysis splitting of water.
  • Pink hydrogen is generated through electrolysis of water by using electricity from a nuclear power plant.
  • Red hydrogen is produced through the high-temperature catalytic splitting of water using nuclear power thermal as an energy source.
  • White hydrogen refers to naturally occurring hydrogen.