In the big Army, when you're deployed for example-- and depending on your unit-- you can purchase some gear such as equipment vests, rucksacks and other bits and pieces. Boots are a big one. At West Point we have to use what we're issued. To my knowledge, if we're talking conventional units, the only time you're not wearing your typical ACU is if you get one of those deployment shirt things. Also, Special Forces don't wear conventional uniforms. The only other thing that could change is wearing MultiCam on deployment vs. regular OCP in garrison. It's kinda different from OCPs but certainly not on the level of "wearing your own camo." I'm sure there are plenty of exceptions on deployment, contractors for example, but as far as I know uniforms are uniform :P