"My skirts are gray."
Translation:Mes jupes sont grises.
The reason is that "marron" is not only a color, it is a real thing (chestnut) which prompts that it remains invariable.
Other color adjectives that do not represent a real thing are standard adjectives, agreeing in gender and number with the noun:
- un chapeau gris, des chapeaux gris (no change from singular to plural, because singular already ends in -s)
- une chaussure grise, des chaussures grises
To know more about invariable color adjectives: http://french.about.com/od/grammar/a/adjectives_inv.htm
Exactly what I wanted to ask -- I got burned on marron, and thought I would make up for it here. Sitesurf's answer was very helpful, too. I studied French for 8 years (it's been a while). I don't recall such a clear explanation of these rules. And I think I may have simply learned these adjectives as rote lists.