"Her shoes are brown."
Translation:Ses chaussures sont marron.
Regarding the lack of a plural form for 'marron,' better French speakers than I pointed out in another discussion that colors don't use the plural form if the color comes from an actual thing; e.g., orange, marron (chestnut), rose. Colors words that are not a thing use the plural form when appropriate; e.g., grises, vertes, jaunes, etc.
If someone knows this "rule" to be incorrect, please correct me. :-)
Please see the explanation just above your question. And here is a helpful link as well: http://french.about.com/od/grammar/a/adjectives_inv.htm. Apparently, I was incorrect in the note above about rose/roses. It is an exception to the rule. My apologies.
Marron is apparently an invariable adjective, like JRPlanet was saying, so it doesn't has to agree with the noun - this link explains :)