Just wondering what the yellow and sometimes red things are at the end of the rifles sometimes used in bct?

By BCT I ume you mean Brigade Combat Team. The colored inserts are fiber optic type front sights designed to gather light. They aid in target acquisition particularly in lower light conditions. The various colors are simply a matter of preference. I've seen green, red, yellow, and orange and the color you choose simply comes down to what works best for you. I really have not seen any rifles with this type of front sight, but I have seen many shotguns.