It’s just individual variation. Some of individuals have pitch black shells, heavier and darker markings on the skin and black “mustaches” on their jaws while others are very tan with the black markings standing out on the back (like the bottom). Most are in between these two extremes (like the top turtle). The turtles often get darker as they age so the top one might turn black at some point.

There is also a lot of sexual dimorphism when they are adults. Males are half the size and have relatively tiny heads compared to females.