Hydrogen can be produced in a variety of ways. Some methods are significantly more sustainable than others, and this is why the processes are often represented by colours. The three most common production methods are called grey, blue, and green hydrogen.
Currently, around 95% of all hydrogen in Europe is produced by steam methane reforming. Steam methane reforming is an efficient process, but unfortunately, quite damaging to the environment. For every kilogram of hydrogen produced with this method, as much as 7kg of CO2 (carbon dioxide) is released.
A slightly more sustainable alternative is blue hydrogen. Blue hydrogen is basically the same as grey hydrogen (produced by steam methane reforming), but the vast majority of the CO2 is captured and stored underground (carbon capture and storage) or reused (carbon capture and usage). Blue hydrogen is considered a lot more sustainable than grey hydrogen, but capturing CO2 requires additional infrastructure and energy. As a result, the cost of blue hydrogen is higher than that of grey hydrogen.