1) Yes, we use feminine forms when talking about colours (colours in general, barva is feminine). But when you're talking about the color of something specific, you use the form corresponding to the grammatical gender of the said object. (Moje auto je zelené.' = 'My car is green.' - Notice the neuter adjectival ending.)
2) There are nouns to describe colours as well. Not used much in common speech, though. https://forum.duolingo.com/comment/24972058
Fair enough, you are right, I'm in the wrong here and I apologize. I could have phrased my reaction more diplomatically and the curt phrasing was caused by some accumulated frustration from earlier on during the day.
Still though, what about the matter at hand? "preferred" vs. "favorite", especially for non-native English speakers?