"White and red are colors."
Translation:Biały i czerwony to kolory.
Colours act like they have word "kolor" hidden somewhere.
All other adjectives need to describe something. If you use words like coś=something, to=it/this , these are neuter so you use neuter forms of an adjective.
Masculine nominative form is the one used in dictionaries.
"Dobra" is a different thing. Used alone means OK/well/fine/I agree in colloquial speech, and I have no idea what is is supposed to describe