The guy who played Harry Potter in the movies most likely wore a jacket or piece of clothing material over him as if it was a cloak, that was the same color as the green screen. When they turn the green screen on, and he is on camera, it will make what he is wearing, match what's on the green screen behind it.
I think that this is a cool idea, especially for soldiers who are fighting for our country and need more camouflage to help them blend in with their surroundings. Hopefully this will really help them, though it would be really cool if they sold them to non-soldiers too just for fun! :)