The real Question should this be, "What Colors do I like?" look and see the colors then whatever color you like paint it that way, everyone has different taste, I have 2 Ideas myself (DON"T TAKE THE IDEAS LOL) that Purple Skin Yellow Scales, and Red Skin Green Scales. Also if you need help with some colors go to color wheel works wonders!
No colours are wrong. You can paint anything you like! What's fun to paint, what appeals to you. Some sample colours: http://images.google.com.au/images?um=1&hl=en&q=frog+poison