Green hydrogen is made by electrolysis of water using electricity from renewable energy only;
Gray hydrogen is manufactured by thermochemical processes using fossil sources as raw materials (coal or natural gas);
Blue hydrogen is manufactured in the same way as gray hydrogen, except that the CO2 emitted during manufacture will be captured for reuse or storage;
Yellow hydrogen, more specific to France, is produced by electrolysis like green hydrogen, but the electricity comes mainly from nuclear energy.
Ademe recently suggested changing the terminology. Hydrogen which was previously called “green” is now called “renewable”, “grey” hydrogen becomes “fossil”, and finally, “blue” and “yellow” hydrogens are grouped together under the name “low -carbon”.