Because the douchey kind of guy who does a mass shooting is the kind of guy who doesn’t want to carry around a barbie pink gun, or one with hello kitty on it. All guns have to be regulation colors, unless they’re collectable antiques, in which case you have to have a special permit.
This way, the crazies who are stock piling weapons for the day the socialist revolution they irrationally fear comes to pass won’t be able to worry about the government taking their guns away, they might whine about not being allowed camouflage, but seriously.
If you’re using it for hunting, most animals don’t see colour and won’t be disturbed, with the added benefit that colors like hot pink are visible from a distance and will help prevent hunting accidents where one human shoots another having mistaken them for an animal.
It would discourage guys who are trying to compensate for their perceived emasculation from trying stuff.
While I would ideally like to strengthen licensing laws (it should not be easier to get a gun than it is a car), I do think this is a measure that everyone can agree on, even the adamantly pro-gun lobby.