Every species paints its ships grey. The Klingons and Romulans don't think their ships are green, they paint them grey.

The only polity that actually tries to make its ships dull grey to every sentient species is the Federation.

Starfleet's "Vantagrey" hull paint is the product of two centuries of research and development, containing a nanopigment which can be wirelessly updated with the very latest in greyness.