Comment 4 for bug 926144

This is more than likely a Unity compiz-plugin issue and I'll have to look at your and my logs more but this is certainly confirmed and it's confirmed that the fix is to set the colour to anything but RGB 0,0,0 (example: I used 1, 1, 1 and could set the opacity to anything above 50 and it would work right -- anything below would result in white.)