"The brown cat is eating the red meat."
Translation:Le chat marron mange la viande rouge.
I may be wrong, but I believe "le chat marron mange de la vianda rouge" would translate as "the brown cat is eating SOME red meat" or simply "the brown cat is eating red meat".
As bisousethiboux pointed out, when you say "THE red meat", it refers to specific piece of red meat, as in the red meat on the floor or the red meat on the table.
It certainly is a specific piece of meat, we know that because of "the" in "the red meat". That's why we have to use "la viande rouge".
If we were talking about the same cat eating red meat in general, because it just likes this kind of meat, we would not have "the" before "red meat". And then, in French, we would say "de la viande rouge".
The adjective "marron" never changes its spelling, not even for plurals. Other adjectives, usually colors, that are based on objects also never change their spelling. (Orange & rose are two other examples of this, and the abbreviation "sympa" does not change, although "sympathique" can add an s)
I'm very certain I've seen them write "roses" for pink (plural) here on Duolingo. About.com confirms: http://french.about.com/library/begin/bl_colors.htm
As a general rule the adjective always comes after. B.A.N.G.S are the exception.
Erm - because "brun" IS brown in French (as well as spanish) http://dictionary.reverso.net/french-english/brun I think the difference is that brun in French French is reserved for hair / fur colour whereas marron is brown objects that aren't hair. In Canadian French, I believe that brun is used much more commonly and when I was at school in England, we were taught brun as brown and not marron (probably a lazy teacher).
For example, on this site: http://french.about.com/od/grammar/a/de-vs-du-de-la-des_2.htm
This is deemed correct: Nous avons mangé de la glace.