Why is "tes chiens sont noirs" correct but "ses chaussures sont marrons" is marked wrong. The correct answer is given as marron (no s)?
Thanks ano. Your response led me to do an on-line search and I found that most of the colour-adjectives derived from nouns do not change in the feminine or plural. Une ceinture marron, une chemise lilas, des jupes saumon...etc. but of course there are exceptions which need to be learned ! Rose is an exception: it changes.
(Edit - cross posted with edythuk - but will leave this post.) Just to add to anomalocaris' answer, if you don't have a dictionary handy, the colours that are named after thing that are that colour don't generally change.(Rose is the one that caught me out straight away after I was given this tip - it does change.) They are marked "inv" or "invariable" if you have a dictionary handy.
emeraude - emerald
lavande - lavender
lilas - lilac
chair - flesh coloured
Source of information: http://french.about.com/od/grammar/a/adjectives_inv.htm
"Marron" is a word that doesn't change form--the masculine, feminine, and plurals are all the same. (I'm sure there are others but I actually can't think of any at the moment...) In general, though, you've got the right idea--colors should agree with the nouns they modify just like any other adjective.