I certainly didn't mean it to sound like you didn't know what you are doing. The first thing that came to mind with you description of dark color and pink belly was shedding. You'd be surprised how many people miss the pink belly when their snake sheds.
As far as just getting darker, I have heard of some radical color changes before, no one seems to be able to pinpoint the source, however diet is strongly suspected. The snakes in question usually went back to normal after a few sheds.
I have also heard of IMG snakes. These animals have a dramatic increase in melanin production that makes them turn dark brown/black.
Gale