Hi Bris,
Such visual style changes are best handled by CSS, and the :hover pseudo-class. And making a server round trip for mouseover will be quite slow and feel unresponsive for this sort of thing.
So use a custom theme and and additional style names for the component you want to set the color on hover.
// Java
myLayout.addStyleName("foobar");
// CSS
.foobar:hover {
background-color: #ddd;
}