I have simply colored in the top of my cards with various colored permanent markers. Mine are all plain black cards with white markings, which you can still read just fine, they're just not white anymore. Then, on a sensibly sized piece of paper that I keep with the cards, I've made a key. The blue one is my full desktop, the red one is RetroPie, etc. You can write this on the adhesive-backed section of a trimmed down sticky note, and keep it in the recessed area on the back of a Micro-SD card holder like this one (https://www.amazon.com/Micro-SD-card-ho ... B006J73RAI
A quick glance at the key, and I can then just grab the Orange one, for example. I can also re-write the sticky note any time I want to reassign my cards to different builds.