"The man has brown shoes."
Translation:L'homme a des chaussures marron.
It's not just a Canadian thing. In French class at school, we were taught "brun," and my teacher only mentioned "marron" in passing as another possible translation. And I know for sure the curriculum we use is based on French French, not Canadian French. So yeah, I really think "brun" should be accepted on duolingo.
many many adjectives and colors are invariable and a long list is here: https://www.thoughtco.com/invariable-french-adjectives-1368796
In French nouns almost always need an article or other determiner.
The English sentence means "the man has some brown shoes" but "some" is optional in the English sentence so can be left out without changing the meaning.
However the equivalent of "some" in French cannot be left out. In this case "des" = "some".
There is no 'marrons' or 'marrones' in French adjectives. That's what I mean by invariant; it doesn't change for gender or number. http://french.about.com/od/grammar/a/adjectives_inv.htm
https://www.thoughtco.com/invariable-french-adjectives-1368796 for the long list of exceptions so helpful if end-changing primary colors are not accurate
One can use the forms "marrons, marrone," and "marrones." However, these would mean an entirely different adjective. This second, different usage is specifically an adjective meaning "illicit or crooked (of professions)," and comes from Spanish "cimarrón." An example of its usage is "médecins marrons (crooked doctors or quacks)," but I don't know if this adjective is still common in modern French or not. Plus, it wouldn't make sense in reference to shoes.
Perhaps I could point out that, despite the usual invariability of 'marron', Linguee does give an inflected example of its adjectival use: 'ses yeux étaient marrons'. Unless Linguee is glossing an error, I think that Duo should accept 'marrons' when the qualified noun is plural.