'Selected' thumb for Photo Gallery?


On the Thumbnail gallery I dont want the thumbs to be images - just blocks of color. So ive assigned the thumb to be a block color jpg for each image.

That is fine but the problem im having is that I want the selected thumb to be highlighted (ie stay at 50% alpha - like the rollover) so you know where you are in the array of thumbs.
Any ideas how I might do this?