It is tough to decide which colors will get which treatment, but in general, lighter colors in the photo will end up with thinner amounts of pumpkin flesh left behind. The exception is usually the iris of the eye which is often left to "glow" by removing all of the pumpkin flesh. Try to think it through.
Start carving. Try to remember that once you have removed a chunk of skin or pumpkin, you can't put it back to where it was.