Hydrogen itself is a colourless gas but there are around nine colour codes to identify hydrogen. The colours codes of hydrogen refer to the source or the process used to make hydrogen. These codes are: green, blue, grey, brown or black, turquoise, purple, pink, red and white.
- Green hydrogen is produced through water electrolysis process by employing renewable electricity. The reason it is called green is that there is no CO2 emission during the production process. Water electrolysis is a process which uses electricity to decompose water into hydrogen gas and oxygen.
- Blue hydrogen is sourced from fossil fuel. However, the CO2 is captured and stored underground (carbon sequestration). Companies are also trying to utilise the captured carbon called carbon capture, storage and utilisation (CCSU). Utilisation is not essential to qualify for blue hydrogen. As no CO2 is emitted, so the blue hydrogen production process is categorised as carbon neutral.
- Gray hydrogen is produced from fossil fuel and commonly uses steam methane reforming (SMR) method. During this process, CO2 is produced and eventually released to the atmosphere.
- Black or brown hydrogen is produced from coal. The black and brown colours refer to the type bituminous (black) and lignite (brown) coal. The gasification of coal is a method used to produce hydrogen. However, it is a very polluting process, and CO2 and carbon monoxide are produced as by-products and released to the atmosphere.
- Turquoise hydrogen can be extracted by using the thermal splitting of methane via methane pyrolysis. The process, though at the experimental stage, remove the carbon in a solid form instead of CO2 gas.
- Purple hydrogen is made though using nuclear power and heat through combined chemo thermal electrolysis splitting of water.
- Pink hydrogen is generated through electrolysis of water by using electricity from a nuclear power plant.
- Red hydrogen is produced through the high-temperature catalytic splitting of water using nuclear power thermal as an energy source.
- White hydrogen refers to naturally occurring hydrogen.