Is it a convention to use the feminine of the adjective to describe a color? Because in french we have a noun for the color as well as an adjective, but I guess in czech we use the feminine adjective?
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