I think I have a faster solution than Rubio. (Or I did, when Rubio's solution took one day longer than mine; he's since incorporated my solution into his answer).
We assume both have at least one bar on their window (or the window couldn't be said to be barred, and they're told their windows are the only barred ones).
We further assume ...
This is an attempt to prove that there is only one possible setup, namely the one that Beastly Gerbil describes in their answer.
First, a couple of observations:
That leaves us with
Two colors AABB:
We have five statements to process:
"Two of us are truth tellers".
"None of us are truth tellers".
"Three of us are truth tellers".
"Only one of us is a truth teller".
"Three of us are truth tellers"....
Along the lines of Glen O's answer, this answer attempts to explain the solvability of the problem, rather than provide the answer, which has already been given. Instead of using the meta-knowledge ...
Say I'm one of the residents of this island, and the Guru hasn't shown up yet. I'm a perfect logician, like all the rest of the residents... but I'm very forgetful. So, to help me remember things, I ...