Colours dull and you might notice the eyes going cloudy - then colours and eyes return to normal for a few days - then the shed.
I've never noticed belly scales being iridescent as part of the process, but then I've never looked!
What they look like will depend on what morph they are. As you can imagine, trying to spot a white-based morph going into the cloudy "blue" phase, can be a bit of an issue!