Two ways to color sort hydrogen have become popular. Although this post was originally developed with the physical colors that hydrogen fluoresces in mind, the community has since developed a color coordinated renewability scheme which is shown after the physical color scheme.
Hydrogen has signature colors that appear when the atoms are excited. This image from NASA-Ames shows a hydrogen arc lamp fluorescing:
Neils Bohr used the emission spectra of hydrogen to develop his model of the atom. In short, quantized energy levels release specific bands of light with unique colors. A description of the physics is here. Here are the corresponding colors and electron transitions:
I used this handy converter to go from emission wavelength to RGB color code. It’s too bad the folks developing the renewability color scheme didn’t follow the above.
Green hydrogen is hydrogen derived without any CO2 emissions. Blue hydrogen is created with CO2 but the CO2 is sequestered, stored, or convertered in some way. Grey hydrogen is produced with CO2 emissions, typically from Steam Methane or brown coal refining.