Hmm, well, Cyrano Jones's scout
was white with red trim. Harry Mudd's ship
in TOS Remastered is grey-hulled, and it's the same model as the remastered Aurora
. The original Aurora
, the one kitbashed from the Tholian ship and Klingon wings, was also gray; it had a reddish tinge, but I think that was illumination from outside.
I'm not sure about Federation civilian ships in the 24th century. The only one I can think of offhand is the Raven
, but its design (and the reference to it with a USS prefix in one episode) suggests it was of Starfleet origin.