I think you'll find he lightens up quite a bit if you sit him in a tub of warm water and scoop water over his shell for several minutes, then take a soft brush and some baby shampoo and give him a good scrubbing, including his front legs.
New growth is always dark like that. He will lighten up with wear and sunshine over time. Look at these two:
Same age, but the lighter one is two inches larger and spends more time in the sun, while the smaller darker one hides underground in a burrow or the night box more of the time. In the pic, you can clearly see how the newest growth is all dark while the older growth has been "bleached" by time and the sun.
My little guy Leo started out pretty light when I got him but all of his new growth is dark. I heard sun fades the dark parts so we will see if that will make a difference since winter is just about over and I’ll be having him spend more time outside. Leo before and after below