Tim -
You can put an event in the MouseOver section for a button, but you can't change its colour (I don't know why, but Microcoft don't provide a property for that. You can change the colour of the text, though.
A better effect can be created by making the button invisible and putting the caption on a label placed behind it. That way the user appears to be clicking on a word, though they're actually clicking your invisible button. (If you want a picture to click on instead, then use an unbound image box instead of a label).
Using this method, you can change the Special Effect on the label, say from Flat to Sunken. It looks quite good. In order that it doesn't stay that way you'll need another procedure attached to the MouseOver event of the Detail section, resetting all such buttons to Flat (or whatever they were before). Of course, the Special Effect isn't the only thing you can change - you can change colour, font, caption, whatever you like.
Hope that helps.
Best regards
Paul