The colors of hydrogen: Why do we care?

Hydrogen is a colorless, odorless gas that burns with a near invisible flame. So why do we constantly hear about green, blue, grey and other colors of hydrogen? What are these colors for? 

• Green hydrogen is produced without any greenhouse gas emissions. It is made by using electricity from renewable sources, like photovoltaics or wind power, to electrolyze water. Electrolyzers use an electrochemical reaction to split water into its components, hydrogen and oxygen. 
• Blue hydrogen is produced from natural gas using a process known as steam reforming, where natural gas and steam react to form hydrogen, but also carbon dioxide. To make hydrogen "blue," the carbon dioxide must be captured and sequestered. If the same process is used, but the carbon is not captured, it is called grey hydrogen. 
• Black and brown hydrogen are made through partial oxidation gasification from black coal or brown coal (lignite). This is the type of hydrogen that creates the largest amount of environmentally damaging by-products.
  
• Red (also known as pink or purple) hydrogen is generated using electricity from nuclear energy. Just like green hydrogen, an electrolysis process is used. The difference is that the nuclear waste is created as a by-product of these processes. There are also some ideas to use the high-temperature reactors or available steam.
  
• Turquoise (or cyan) hydrogen is made by a process called methane pyrolysis. The by-product is solid carbon. Depending on the thermal process that is used for pyrolysis - for example, whether it comes from renewable sources - and the capability to store the solid carbon permanently, this can be a low- or no-carbon process.
  
• Yellow hydrogen is produced by electrolysis directly from solar energy without the intermediate step of creating electricity. In some publications, the term 'yellow' hydrogen is used when the electricity for the electrolysis process comes from multiple sources, some of them renewable, some of them conventional.
• And lastly, white hydrogen, is naturally occurring geological hydrogen. Yes, there is a process that involves drilling a hole in the ground to get to hydrogen, with some fracking involved; however, there is currently no large-scale exploitation of this relatively rare resource 
  

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