You could wash the shirt in a product called "camo restore" to darken it. I used camo restore to bring my old faded Brigade Quartermasters ASAT back to the original colors.
That said, camo is vastly over rated, you don't really need the stuff. I hunt some of the spookiest deer on the planet and only wear shades of grey and brown, no camo. The deer don't pick me out any more than when I was a camo devotee.
After 40 years of wearing camo, I literally felt naked going into the deer woods without it but was determined to break free of the camo clan. It didn't seem right for me to hunt with my 30s and 40s style archery equipment and wear the latest high tech camo clothes. Sorta' like putting a scope on a flintlock rifle. I had to have the right persona to match the equipment.