This little trick won’t solve global warming or anything, but it is a durable way to color-code keys.
I’ve used those vinyl key wrappers, but they make the key fatter and I carry plenty of keys. Plus, eventually they come off. Of course you can put colored nail polish on the key but it chips off. Here’s a simple mod:
Drill some dimples onto the key surface. Put some colored nail polish into the dimple. When it dries, put a layer of clear nail polish on top of it. The dimple acts like a little reflector. And because it is recessed into the key surface, it is not subject to so much wear, and won’t chip off.
Once upon a time, stuff like this was called “helpful hints” but today we call them “Life Hacks”.