I honestly find it kind of annoying how it's not possible to paint any of the cannon barrels. I think a lot of vehicle creations would look a lot nicer if you could add a custom color to the barrel and any barrel extensions or muzzle brakes. For example you could match the color to the turret or make a fancy stripped pattern.

One possibility could be to use the additive paint layer so that the base of the gun can be a different color; I feel like this would help with backwards-compatibility as well.