For the 516 Assault Rifle, the paint shown is whatever color the receiver is(receiver should also be added to all weapons as an option/part you can paint...).
When you select the Barrel when in Gunsmith>Paint for the 516 the part itself isn't even shown. For such a coveted weapon in the game some extra TLC was expected but considering the enormous list of issues with this game and the obvious problems that "somehow" slipped by the playtesters Ubisoft hired(though it's standard practice at this point to just let the consumer pay for broken games and let the customer playtest for you now...), it's no surprise at this point that yet another issue like this popped up.

Also there is no front sight post for the M1911 Promise handgun when in ADS(wasn't this part of our Ultimate Edition purchase?...).