Andy Probert always balks that every spaceship should be gray, and I agree with him. The workbee is yellow because it's construction equipment, like a bulldozer, steamroller or steamshovel.
There's no reason why Starfleet or anyone else couldn't have leaped out of a Chris Foss painting, especially since that would make visual
ID a bit easier.
The NX's bronze color scheme goes a long way in selling the ship as something at least a little different from familiar Starfleet, which is why I'm kind of sad that the ENT Netflix campaign seems to already be dropping it for the newer Earth ships.
Heck, as much as I may question the aesthetics of Doug's refit design, it's definitely better in bronze than silver or grey.