Brown: Hydrogen produced from coal gasification
Grey: Hydrogen produced from natural gas through steam methane reformation, without any mechanism for carbon capture
Blue: Hydrogen produced from natural gas with a mechanism for carbon capture
Green: Hydrogen produced through electrolysis from renewable or other carbon-neutral sources.
Other than the above, other types are 'Yellow hydrogen,' which uses nuclear electricity for electrolysis. 'Turquoise hydrogen' is produced from by molten metal pyrolysis of methane, with solid carbon byproduct.