Colors and Number
Marron is the same in the masculine singular and plural, as well as the feminine singular and plural? I thought an -s would be added in the plural. Perhaps marron is an exception?
Yes, it is an exception. When color adjectives are also nouns refering to real things (marron = chesnut), there is no agreement in gender or number.
please read this for more info : http://french.about.com/od/grammar/a/adjectives_inv.htm