Thanks for your suggestions everyone.
As I already have quite a few fish with yellow, purple, and black, I wanted something that had a nice contrast with the other fish and the corals (most of which are red/pink).
Over the weekend I found Stu, a blue sapphire (aka springer's) damsel. He is a beautiful electric blue with black markings. The picture doesn't even come close.
I chose one of the smallest of the bunch, as my research shows that younger springer's damsels are much more peaceful, while springers as a whole (adults too) are one of the most peaceful of the damsels.
So far there have been no issues with introduction with the other tank mates (unfortunately I hadn't planned to get Stu so early, just happened to find myself in the NE after missing my turn off Deerfoot.... and my QT wasn't set up, with the pump and heater being broken when I tried.) He set up shop around my monti caps, and is small enough that he hides between them and the rock when he is startled.
So far he eats like a pig, and loves to swim around the column.
