My husband and I replaced our phones not too long ago and we ended up getting the exact same one. Short of unlocking them to see the background, there was no way to tell them apart. So I painted the back plate on mine. I sanded and primed it first, then did a base of bright orange. The image is hand-painted on, complete with a nice clear coat to protect the paint job.

The clear coat reacted weirdly with the white acrylic I used and has caused it to crack a bit, otherwise it's turned out pretty well. Now I can identify my phone. :P