Step 1Grab an iMac image and save it somewhere.
Step 2Load it into The Gimp, and inverse the colors: Filters-->Colors-->Value Invert. We're almost done already!
Now all you do is UN-invert a few things, and you're done! Now seriously, why would you want to waste all that time in Photoshop, when you can do the same thing a hundred times faster in The Gimp!? That's the power of OpenSource! YeeeeeAwwwwwww!!