Hydrogen (H) is the lightest element of the periodic table and the most common substance in the universe. It can be used as feedstock, fuel or energy carrier and does not emit CO2 when burnt, that is why you often hear about its high potential for decarbonizing our economy. But we don’t want to decarbonize our atmosphere in total, we only want to come back to a CO2 ratio in the atmosphere like is was around 1900. What we really want to achieve is a defossilizing of our planet, which means to eliminate the burning of fossil fuels.
In nature, we find it mostly in gaseous form (H2) and it is colorless. That is why, when you hear about “white hydrogen”, we refer to the naturally occurring one that might be (rarely) found in underground deposits. We don’t have any viable strategy to use these deposits now, so we apply different processes to generate it artificially. That is what the colors are for: each one refers to the energy source and/or process that was used to produce hydrogen