[2008] highlighting colour on buttons

Question 1:
Basically I want to know how to highlight buttons, in some programs you move your mouse over a button, it colours it, I understand that, but what I do not get is I have seen the colour of the image on the button sort of 'highlight' like in mozilla firefox you hold your mouse over the Refresh button the colours get deeper and darker, and it looks like it's jumping out at you,
I am wondering how I can do this.

Question 2:
I have also seen a gradient effect applied to a button, I am wondering how I can do this aswell?

Question 3:
If I draw onto a button, will it draw over any background images/images/text I have on that button?