Highlight Outline on Mouse Enter

[static_recharge]

So I thought this would be easy to do, and this is not my first UI project, but I can't seem to figure this one bit out. I have a grid of coordinates that I want the user to select a square from. When they mouse over a square I want the outline to change colors showing their selection and then obviously clicking on it will activate my function for what to do with that information. I can't for the life of me figure out how to create the outline effect that I'm looking for though. Any help would be appreciated!