From black hydrogen, linked to fossil fuels, to green and the promising turquoise hydrogen, the “rainbow” of hydrogen describes the different pathways toward more sustainable energy. Beyond colours, strict criteria are needed to measure environmental impact and understand which will truly become the fuel of the future. Meanwhile, white hydrogen is emerging on the horizon.
A colourless but not harmless gas
It is paradoxical to speak of a hydrogen rainbow, since hydrogen is a colourless, odourless, and tasteless gas. These properties, which make it appear “pure”, also create technical and safety challenges: its molecule is so small and volatile that adding odorants is difficult, while its nearly invisible flame can pose risks in domestic applications.
Hydrogen is still considered a future fuel due to its high energy content and the fact that it produces only water when burned. However, not all hydrogen is “green”, as its environmental footprint depends entirely on how it is produced.
The hydrogen rainbow
To classify production routes and environmental impacts, science and industry introduced the “hydrogen rainbow”, a colour-based system not yet formally standardised but useful for understanding sustainability levels.
Black or brown hydrogen comes from coal gasification and is the most polluting. Grey hydrogen, produced via methane reforming without carbon capture, accounts for more than 95% of global production according to the IEA.
From fossil to “clean” hydrogen
Blue hydrogen uses carbon capture and storage technologies. Turquoise hydrogen is produced via methane pyrolysis, splitting methane into hydrogen and solid carbon that can be reused.
This technology is still experimental but promising, especially for battery and conductive material applications.
Green hydrogen is produced via electrolysis powered by renewable energy sources and is considered the cleanest option, though still expensive.
Nuclear and solar variants
Pink, purple or red hydrogen comes from nuclear energy. Yellow hydrogen is based on solar power, while orange hydrogen is an emerging experimental route using plastic waste.
The mystery of white hydrogen
White hydrogen is a natural form found underground, generated through geochemical reactions. Its exploitation is still at an early stage.
Beyond colours: measuring sustainability
The colour system is useful but simplified. Two hydrogen sources with the same colour may have very different environmental impacts. Organisations such as UNECE are working on stricter, standardised sustainability criteria.
Conclusion: beyond the rainbow
Colours provide orientation, but are not sufficient to describe the full lifecycle impact. The goal remains truly sustainable hydrogen as a key pillar of the energy transition.
