Translation:You are the girl with the red dress.
Nobody know why Duo is doing this. When I first started Duo 2 years ago, col was rarely used. Then, after Duo's fairly recent change in format to the "crown" tally, instead of "fluency", col appears with a lot more frequency. I can't recall whether Duo even uses con il any more, which seems awfully strange, given the number of comments which say that it is archaic (though of recent vintage, I think).
It may be as simple as the moderators now being comprised of old Italian professors of language who prefer col. I was discussing a topic in Spanish with a retired college professor about a different subject, but what he had to say rings true here:
"Grammar rules are generalizations written down by grumpy old, literate teachers who are upset that people aren't saying and writing things the "correct" way, that is, the way it used to be. Language is always changing, first as spoken, then, slowly, as written."
In England we would often say. The girl with the blue dress, him with the black jacket etc. It implies they are wearing it. It could be completed by saying The girl with the blue dress on. What fascinates me is why the preposition col is no longer used. It seems logical to use it given all the other prepositions we are taught.