"Are you painting a house?"
I'm betting it's a consequence of the way optional words can be handled in the Incubator. I think contributors can indicate that both "Egy házat festesz?" and "Házat festesz?" should be accepted by including the first sentence in the translation list but putting "Egy" in parentheses or some such. Normally, this doesn't really matter because Duolingo disregards capitalization in checking answers. However, it does matter when you miss a sentence, and the system shows you a correct one, as it shows what's literally typed, which for things like optional articles indicated with the optional word syntax is always going to have wrong capitalization in certain cases. Most courses just capitalized the optional first word, usually a pronoun, leaving uncapitalized sentences to be displayed. The Hungarian course in certain cases at least seems to have taken the other tack.
I'm not sure what they wanted to emphasize, the thing is "festel" is quite rare. By rare I mean I'm not sure I've ever heard it being used. At the end of the day, I probably have, in speech, that's what "colloquial" suggests after all. But still, I think google is representative with this, giving me 32 Hungarian results for "mit festel" and about 2900 results for "mit festesz".