Translation:What is your favorite color?
Favourite is an absolute (non-gradable) adjective, so something cannot be more or most favourite. - Stickler
"What is your most favorite color?" would be accepted. Reported on Nov. 18, 2017.
"Most favorite" is usually considered redundant, particularly when "favorite" is already taken to mean "most favored". Although it's not unheard in native English as an emphatic statement, Duo probably won't allow it here.
An exception to it being redundant might be if you say "X, Y, and Z are my favorite foods, but my most favorite is Y". It might still rankle some sticklers, though.
However, in spite of the common assumption that "favorite" means "most favored", some dictionary definitions don't actually attach the notion of the superlative or the absolute to "favorite", instead defining it along the lines of "treated or regarded with special favor or preference", and not "treated or regarded with more favor or regard than anything else in its class". Merriam-Webster.com and Dictionary.com are two examples.