The Hydrogen Case Confusingly for a colourless gas, the convention is to use colours for hydrogen produced by different processes. Firstly, as mentioned, the existing grey hydrogen. Secondly, we have blue hydrogen, which would also be produced from natural gas, but with theoretically limited emissions as production would be combined with the much hyped technology of carbon capture and storage (CCS). Finally: green hydrogen, which is produced from the electrolysis of water using renewable electricity such that the net carbon emission is definitely close to zero. So why are we not immediately switching from fossil fuels to blue or green hydrogen? This is because using hydrogen as an energy source also has numerous disadvantages. First of all, for applications like mobility and heating, the required changes to the existing infrastructure will take a long time, cost a lot of money, and it’s not clear to investors how big the actual market will be. Secondly, applications of hydrog