It refers to how the hydrogen is produced. “Green” hydrogen is produced using a electrolysis process powered by renewable energy. “Blue” hydrogen is created from a chemical reaction involving natural gas, and the CO2 from the blue hydrogen process is then captured and stored, making it theoretically carbon neutral (however, it is criticized for the inevitable methane leaks and such from the process). “Grey” hydrogen is the same as blue hydrogen but the emissions aren’t captured and stored.