is there a way to color the entire image (let's call it the pallet)

Please let's not call it that. The term palette has a small number of very specific, related meanings within the realm of digitally generated images. It's a term which everyone in the field uses in those contexts. If you suddenly start to assign arbitrary new meanings to words which everyone else understands to mean something different you will only create problems for yourself. Instead, consider "image" or "canvas" depending on the subtleties of what you refer to.

If you are new to computer generated images as well as to GD::Simple then you will have to learn the jargon as with any other new field. Good luck with your new endeavour.