No standard colors (as far as we know it!) did exist. Since it was up to the individual ships' captains to equip and outfit the vessels, it is very close to assume that they could also influence the decor of the ships. In fact, it appears that, with few exceptional exceptions, the ancient mariners did not use that many colors, nor stark ones. Shields which usually covered the upperworks/ gunwale of the galleys might have been tribal, or in Greek and Roman navies based on demes and centuriae. We just don't know for sure! Since the ships themselves were tarred and pitched, it is probably quite safe to assume that their hulls generally were in a darker, almost blackish shade of brown. Cedar and elm were very much used in ship building, so the nuances could be seen by detecting what kind of wood the ship was made of. So, the way you have it, I think is a safe way to depict it.
Hull designs next. This is a far more complex subject since there's some controversy still about exactly how the hulls might have been designed. There appears to have been several 'schools' though on how to build the most agile, fastest, or most powerful or strongest vessel. The principal proponents seem to have resided in Athens, Corinth (where the Four was invented!), Syracuse and various Phoenician cities, chief among them Tyre, Sidon and, of course Carthage. The differences appear largely to have been about such issues as the curvature of the Aphlaston in the stern, the true length/beam ratio (determined by the decked-over sheds) height of oar compartment, distance between oartholes and how wide and deep the outrigger box (paraxeiresia) should be, with the Phoenicians evidently favoring a very shallow, slim structure, whereas the Eastern Greeks favored a more substantial deal, and, finally the Western Greeks and Romans went somewhere in-between. The facts are, however, still very scant and, in many instances, severely contradictory, to enable any fail-safe conclusions to be drawn!
My Avatar:Петр Алексеевич Безобразов (Petr Alekseevich Bezobrazov), Вице-адмирал , царская ВМФ России(1845-1906) - I sign my drawings as Ari Saarinen