"Red and blue are colors."
Translation:Vermelho e azul são cores.
If one is discussing colours, why is "vermelha" not correct? "cor" is feminine, so should one not use "vermelhA"?
Yes, but you are not using "vermelho" as an adjective for "cor", it is a noun in this case so it should be masculine. It would be correct in the sentence "a cor vermelha", but in this case "vermelho é uma cor".
O vermelho e o azul são cores. Seems like that should be okay. Am I wrong?
It's possibly nitpicking, but the sentence asked for "Red and blue", not "The red and the blue". There is usually very little lenience with articles; "um" and "o" are not interchangeable. I guess "o" and the absence of an article are not interchangeable, either?
In portuguese it's very common to use articles where english doesn't. So "o vermelho e o azul são cores" should be right too.
So in conversation, if I were to say vermelha in this context instead of vermelho, I would sound like an uneducated illiterate?
Yes, vermelha doesnt exist in any context unless something like reddish- as in AVERMELHADA