Need help randomly switching backgrounds

So I have a file that right now when you click on the navigation is loads a different color background. But as you’ll notice, they all actually default to the blue background and then scroll the new one over it. Right now each page (print, web, etc) are fixed with scrolling in a specific new background. What I would like to do is make it somewhat random. I have 4 different background colors and what I would like to do is whenever you click on one of those main page links it loads the next color in the rotation. For example, the “print” page could actually be any of the 4 colors…just depends on when you click on it.

See linked file for a bit of an explanation.

http://www.joshpigford.com/misc/flash/interactive_portfolio.html