The other week I was spending the night at my cousin's house. In that time she lent me her key which i attached to the rest of mine. Unfortunately, both of our keys looked the same and I accidentally gave back the wrong key! If only I had thought of this DIY sooner, the mix-up wouldn't have happened.
This is a really great way for personalizing your own keys so there won't be a mix-up.
Tip: For a clean line, you can use tape to mask off the key before painting. (I didn't think about this until later!)