They found preserved melanosomes. It was previously thought these were the remains of bacteria.

You can find details here, but I highly recommend reading Dinosaurs: New Visions of a Lost World by Michael J. Benton, who was one of the ones doing the research. The book is very fun to read (he's got that typical dry british humour), does a great job of describing the history and current status of paleontology (which is apparently exploding in new discoveries right now) and it has absolutely lovely illustrations, including the one I linked above. And also this anurognathus that is the cutest thing ever: