I have seen many that didn't have any black or red on their plastron. just pure yellow. But these could be hybrids of fl. redbelly and fl. cooter. I think nowadays these are more common than pure fl. redbellies. Northern redbellies have alot of blach and bright red bellies. And maybe since where they come from there aren't other cooters so they only breed with their species.
:ITs a Florida Red-Belly. contrary to what the other poster said, they do have black on their plastrons, although usually not that much. .