Hydrogen is an invisible gas, yes. But different forms are given color codenames to help distinguish among them, essentially based on the molecule used to produce hydrogen and the source of energy. There is no universal agreement on what the colors mean, so definitions may change over time or between countries. Here’s our guide to the generally understood hydrogen rainbow:

GRAY
The most common form of hydrogen production – roughly 95 percent – is produced today from the main component of natural gas (methane) through a steam reforming process. In this case natural gas reacts with steam at high temperatures and pressures producing hydrogen gas and carbon dioxide (CO2). CO2 is released into the air, accounting for 2 percent of the world’s CO2 emissions.

BLUE

Blue hydrogen is produced by the same steam reforming process as the gray hydrogen. In this case, carbon capture and storage (CSS) is added in its production to avoid the CO2 emissions.

BLACK and BROWN
These are the most environmentally damaging forms of hydrogen because they’re created using bituminous coal (“black”) or lignite (“brown”). Gasification byproducts CO2 and carbon monoxide are released into the atmosphere.

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GREEN
Made with surplus energy from renewable energy sources such as solar and wind power to split water, green hydrogen produces no harmful greenhouse-gas emissions, just hydrogen and oxygen.

PINK , PURPLE or RED
These colors denote hydrogen that is produced using nuclear power as the energy source to break the water molecule into hydrogen and oxygen.

TURQUOISE
The newest color is produced by a process called methane pyrolysis, which creates hydrogen and solid carbon. It is still experimental. If the process is powered by renewable energy and the carbon is used or permanently stored, turquoise is potentially a valuable low- or zero-emission hydrogen.

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YELLOW
Yellow hydrogen is a new term to define hydrogen produced from the electrolysis of water using solely solar power as the energy source. It is a particular case of green hydrogen.

WHITE
This form of hydrogen, not very common, is naturally occurring in geological deposits, generated by the interaction of water with some metals of the rocks at high temperatures and pressures. It can be released by a process named fracking. The same name is given to the hydrogen produced as a byproduct in industrial processes.