This life hack is seriously the most simple and genius thing ever.
I did this long ago to a set of keys that looked virtually all the same for my old apartment. That was a couple years ago, and I had completely forget about this trick until recently when I realized I was constantly getting frustrated not being able to tell two current keys apart (first world problem I know).
For this DIY all you need are some keys you want to colour code, and fun nail polish colours (raid your girlfriends/moms/sisters collection if need be boys)
Paint 3-4 coats on one side of the key. Let it dry completely, and then flip and paint the other side.