I love double exposures taken with different flash colors! The only thing you need for this is a camera with a flash and the possibility for double exposures (MX). Try it! It’s easy and you’ll love the results!
It’s not difficult to try it yourself: take your camera with double exposure and flash, shoot a picture in one color, then change the flash color and take a second picture.
The first time I saw a picture with this technique was in the home from eddie_2k9. I tried it myself, and loved the results! Since then, always when I take pictures with flash, I try to use two flash filters. With the Diana flash it needs some time to change the filter gels, but with a Colorsplash Flash or my Holga flash it’s easy and fast. The faster I shoot, the more overlapping are the pictures. But you will never get a 100% overlapping image!
My favorite combination is red and green, it’s a bit like a 3D picture. But also other colors are nice!