Essentially, you use Rit dye and PBT keycaps for best results. Rit has a comprehensive guide to mixing their dyes to achieve pretty much whatever colors you want.

Carhartt and Rit are partnering to sell do-it-yourself dye kits to inspire creativity towards apparel both new and old. Dye kits can be bought at select Carhartt stores and online at Carhartt.com ...