Why are most spacecraft in movies and TV gray or white?
I can imagine some reasons:
We live in a world where most big vessels, whether boats or aircraft, are gray or white, so designers follow that tradition in imagining space vessels.
Gray and white seem serious, therefore suited for life-and-death conflict, while color is fun. (Until the 1960s, most serious movie dramas were in black and white, while color generally went to lighter fare like musicals and cartoons.)
Besides, gray and white show up better on screen against the black of interstellar space than, say, midnight blue or dried-blood red.
But yellow also shows up well against black, which is why the symbol for “Danger: Radiation” is yellow and black. Pink and pastels contrast nicely with black, too, although I realize that our culture considers pink too girly and sissified, and pastels too soft and ethereal, for villain-fighting action-adventure craft like the U.S.S. Enterprise or the Millennium Falcon.
Still, I wouldn’t mind seeing more splashes of green or orange amid all that gray and white.