The color change of the button shows you succeeded in pushing it. If you don't do this instantly most people are conditioned to try again. This is especially valuable for people with reduced motor control. It is completely independent of whether that push is a useful input given the current state of the software. Obviously when well written software knows it can't accept the input it should have disabled the button, and even moderately well written software needs to provide a near instant feedback that the action is processing or has been cancelled.