Translation:Sometimes she can be completely red.
It is odd out of context. but remember in French the gendered 3rd person pronouns il and elle can also be translated as "it" in English. So "sometimes it is entirely/completely red" is another acceptable translation, and when speaking about other things, it might make more sense. As far as I know this is not an idiom in French for being embarassed, but all the same, I do not profess to be very good with French idioms, so don't take my word on that! ;)
Since Duo gives the translation as "she", it does seem to imply we are talking about a person. And I agree, in that context, the translation is at best clunky.
But there's nothing about the French that requires that it be about a person. You could be talking about a variety of apple, or poinsettia (both feminine nouns). An apple can be red, green, yellow, streaked red and cream, and sometimes, it can be completely red. If Duo doesn't accept "it" instead of "she", it should.