plural for "are" Singular for "is" What colors are the cars? they are red. green. yellow, black and blue. what color is the car? it is white.

Grammatically, colors is correct, but colloquially, it sounds awkward. In real life i think mist people would say "what color are the cars" and if each car's a different color, the listener would still understand that and point out the color of each car individually. If you say, "what color are the cars" and each car has the same colors, but there are more than one color, the listener would most likely point out the most most prominent color (i.e., if the cars were all lie with a white stripe), the listener would likely respond with blue.

The question is usually phrased ‘what colour are the cars?’. The question doesn’t assume that the cars are all different colours, so the colour(s) may or may not be different, and so plural. You could ask ‘what colour are those cars?’ And the answer might be ‘well, one is red, one is white (etc)’, but in the question colour is singular. A little confusing I know but that’s how it’s formulated, in UK English anyway. Nb colour is the UK spelling of color.

BTW What colors are the cars? they are yellow, red and black. You use the American spelling of "colour", yet no comma before "and". I understand that Americans seem to always use a comma. What colours are the cars? they are yellow, red and black. What colors are the cars? they are yellow, red, and black. . I am be mistaken, thinking it is the norm, but that is what I have seen.

Grammatically, colors is correct, but colloquially, it sounds awkward. In real life i think most people would say "what color are the cars" and if each car's a different color, the listener would still understand that and point out the color of each car individually. If you say, "what color are the cars" and each car has the same colors, but there is more than one color, the listener would most likely point out the most prominent color (i.e., if the cars were all blue with a white stripe), the listener would likely respond with blue.