Looks like you have a beautiful gulf coast turtle. I don't see anything wrong with his shell coloring. I think you'll find that the shell color will change as it grows. It may even turn more darker in time. I wouldn't worry about it. So long as he's eating, drinking, pooping, and active, he'd be just fine. Don't forget to give him calcium block. You can put the whole block in the water. And, he will need UVb lighting!