"I have brown eyes."
Translation:J'ai les yeux marron.
There are a few: châtaigne, marron, and alezan. "Alezan" refers to a color, not the nut. A "châtaigne" is the generic word for any random chestnut. A "marron" is specifically the large chestnuts cultivated for consumption or use in food. All marrons can be called châtaigne but only some châtaigne are marrons.
I found a very useful explanation on adjectives for colour: If the adjective derives from a noun (marron, orange, olive) its not adapted to gender and plural. If the adjective is combined with an adverb its not adapted either (des yeux bleu clair. Otherwise its adapted (une chemise bleue, un stylo bleu).