Answer: There are huge differences in shell coloring in any group of Red-eared Sliders, just as there are huge differences in the way people look. There are also geographic variations across their range. Perhaps your sliders descended from a group of turtles from the eastern portion of their range. Not only are the sliders from this area less colorful they also intergrade with Yellow-bellied Sliders. These specimens generally have less-distinct red "ears" and more muted and darker designs on their shells.
Red-eared Sliders from the southwest section (Rio Grande area) of their range are usually the most colorful. This group gradually melds with the Central American and Mexican individuals. They have much more color and bold, well defined green-yellow designs with bright red "ears".
I would also like to let you know that age and ultraviolet light can affect color. Some individuals become almost solid black (melanistic). Scientists feel the melanistic color protects the turtle from natural sunlight.
Congratulations on raising your first sliders. It certainly sounds that you have done a great job!