Those who make fusions take a few parts of some Pokemon and splice it with the Mew. It uses the color scheme of either Pokemon, and, to be honest, is hard to explain. I found a video that can explain better.
I use this for my own fusions. It's quite a nice little guide that gives some tips. It can be Photoshop, or even MS Paint.
Pokemon SoulSilver FC: 1335 1407 6427 When Life gives you lemons, throw them back and demand cookies instead! -Official Sandslash of the PMD3 boards