What do the hydrogen colours (green, blue, grey) actually mean?
Hydrogen's colours describe the carbon intensity of how it was made, not the gas itself. Grey hydrogen comes from steam methane reforming of natural gas and emits about 10 kg of CO₂ per kg of hydrogen; brown hydrogen from coal gasification emits about 20 kg; blue hydrogen is grey hydrogen with carbon capture and storage added; green hydrogen is made by electrolysing water with renewable electricity. About 95% of the world's hydrogen today is brown or grey.
The colour system exists because on Earth hydrogen is a carrier, not a resource. Hydrogen is the most abundant element in the universe, about 90% of all atoms and roughly 75% of the mass of the visible universe, yet almost none exists on Earth as free H₂ — it hides in water, in methane, in biological macromolecules. Getting free hydrogen means first spending energy to unlock it from something else. So how clean hydrogen is depends entirely on that upstream step: split water with renewable electricity and you get a genuinely green fuel; make it by burning natural gas and you get fossil energy in a different shape, with emissions barely reduced.
Grey and blue hydrogen share the same chemistry. Steam methane reforming (SMR) runs in furnace tubes at around 1,000°C, where methane reacts with steam over a nickel catalyst to give CO and H₂ (the endothermic reforming step), then a water-gas shift converts CO into CO₂ and more H₂ (the exothermic shift step). Auto-thermal reforming (ATR) improves on this by injecting pure oxygen into the reactor so partial oxidation supplies the heat internally; the CO₂ emerges from a single concentrated outlet with a theoretical capture rate above 99%, which is what makes blue hydrogen's carbon capture practical. The controversy sits upstream: in 2021 Cornell's Robert Howarth and Mark Jacobson published research showing that once methane leakage across the gas supply chain exceeds 0.2%, blue hydrogen's climate benefit may be worse than simply burning the natural gas.
Green hydrogen is the electrolysis route — zero carbon end to end, and today the most expensive. Saudi Arabia's NEOM project plans 600 tonnes per day of green hydrogen by 2030, among the world's largest single green hydrogen projects. Pink hydrogen uses nuclear electricity for the same electrolysis, also zero-carbon, and nuclear's round-the-clock output gives capacity factors near 95%, so the electrolyser runs far more of the year than it would on intermittent renewables — which matters because electrolysers are capital-intensive equipment that only amortises when running. Turquoise hydrogen takes a different path: methane pyrolysis splits methane in an oxygen-free high-temperature environment into hydrogen and solid carbon black, with no CO₂ emitted, and the solid carbon is a saleable industrial material that helps offset process costs.
One category needs no manufacturing at all. Gold or white hydrogen is geological hydrogen occurring naturally in the crust. In 2012 a test well in France's Lorraine region unexpectedly encountered a large geological hydrogen deposit with proven reserves of about 46 million tonnes; discoveries have since been reported in Mali, Oman and elsewhere, and roughly 20 startups worldwide are now exploring for it. If it can be produced at scale it would be disruptive: production costs are extremely low and it consumes neither water nor electricity. For green hydrogen, cost is decided by power: electricity is 60%–70% of the levelised cost of hydrogen (LCOH), and at USD 20–30/MWh green hydrogen could reach USD 1–2/kg, close to grey. Reality has been rougher — BNEF data for 2025 show installed electrolyser engineering costs in Western markets running over 57% above 2022 expectations, with European green hydrogen still costing USD 4.5–6/kg.
| Colour | Production route | Emissions and key characteristics |
|---|---|---|
| Brown | Coal gasification | About 20 kg CO₂ per kg H₂; the cheapest route and the highest-emitting |
| Grey | Steam methane reforming (SMR) of natural gas | About 10 kg CO₂ per kg H₂; brown and grey together are about 95% of world hydrogen |
| Blue | SMR or ATR with carbon capture and storage (CCS) | Climate benefit may be worse than burning gas once methane leakage exceeds 0.2% (Howarth and Jacobson, 2021) |
| Turquoise | Methane pyrolysis, yielding hydrogen and solid carbon | No CO₂ emissions; the solid carbon black is a saleable industrial material |
| Green | Electrolysis of water using renewable electricity | Zero carbon, highest cost today; Saudi Arabia's NEOM targets 600 tonnes per day by 2030 |
| Pink | Electrolysis powered by nuclear electricity | Zero carbon; round-the-clock nuclear gives electrolyser capacity factors around 95% |
| Gold / White | Naturally occurring geological hydrogen | Lorraine, France, 2012: proven reserves of about 46 million tonnes; roughly 20 startups exploring worldwide |
Hydrogen is the lightest and most abundant element in the universe, but on Earth its cleanliness is not a gift of nature — it is decided entirely by the energy used to make it.
Sources
- IRENA, Green Hydrogen: A Guide to Policy Making
- Robert Howarth and Mark Jacobson (2021), research on blue hydrogen's climate footprint and methane leakage
- American Institute of Chemical Engineers (AIChE), Steam Methane Reforming: Process and Optimization
- BNEF (2025 data on electrolyser installation costs and European green hydrogen production cost)
- The Full Spectrum: Every Energy Source Explained — A Singapore Perspective, Chapter 14.2 and 14.8
This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第十四章 14.2