Jenius,
1. The only way it has worked for me is to really make sure the resin piece is completely mold release free. I used this on a custom VF-1A head that is made up of half rein and half plastic. **TIP: Use ultra-fine steel wool to sand the resin and plastic just a little , prior to application, this will help the Fusion or Plastikote adhere to the resin.
2. It does work on dark colors, but very light coats are required. Read and follow the instructions on the can and you should be albe to reach your desired color. I used white on blue (1/35 Legioss) and got the color to match the white of other white on white parts.