Movie Buttons

I have no idea what I am doing with Flash, but we have a web developer who has created our site entirely in Flash, and one section is our gallery. He has these buttons that will pop up an image inside of a frame on the stage, and it works nicely. The only thing that I want to do with them (and he says he does not know how to do it) is have them change color or give a different look aftre you click them so users know they have viewed those images already. If anybody could point me in the right direction on this or sned me some scripting that would work with this, I would really appreciate it. Thanks!