Hi,
the buttoncolor attribute is for changing the overall color, yes... to change it on mousover only, you'll have to change the interface.fla and publish a new interface.swf. More specifically, the ButtonPage library element. Change the over state accordingly.
That'll be applied to all page buttons, though. If you need it to be specific you'll have to dig into the actual code of the... NavigationBar if I'm not mistaken... and change handle that using an additional event listener for mouseover/out events for the buttons.
Regards,
Florian