On several different occasions on The Big Bang Theory, Sheldon and his pals are seen either playing paintball or going to play paintball. It's perhaps the most manly activity they involve themselves in, so, appropriately enough, they tend to dress the part.
Featured on The Big Bang Theory episodes
S02 E06: "The Cooper-Nowitzki Theorem"
S02 E16: "The Cushion Saturation" S05 E01: "The Skank Reflex Analysis"