You *REALLY* need to learn to use a debugger. Really. Now.
I would recommend using CHROME as your browser. It's the easiest debugger to use.
Just bring up your page in CHROME and then hit the F12 key.
Click on the CONSOLE tab and voila! There is your FIRST error:
function GameOver ();
Oops...what is that semicolon doing there! Kill it quick.
But your colorChange()
*IS* working. The problem is that, since your buttons have no *VALUES*, the color has nothing to affect! Put an "X" (say) as the value of each button and the "X" will change color.
Or try changing the background color.
And why did you choose black for the default? That means that if black comes up the user gets no clue at all, right?