Performing a little bit of thread necromancy since I ran across this and wanted anyone who ran across it in the future to know that, as of Isadora 4, the Button Control has Control Properties for the color of the button in its on and off states:
The method of using a picture is still totally viable, and can be nicer if you want to include a fancier-looking button designed in Photoshop. Also, it's worth noting that you can use images that have alpha and the alpha will be transparent on the button in the Control Panel and you can make some pretty neat buttons that way.