Windows computers have blue screens, black screens, and I've even heard rumors of red screens... of course I've also heard rumors of Bigfoot.
Macs have a gray screen and since I know some of you Windows users, who I normally cater to, also "know people" who own Macs, I thought this excellent troubleshooting guide by our own Tom Nelson, About.com Guide to Macs, might interest you:
Mac Stalls on Gray Screen at Startup: Troubleshooting Mac Startup Problems
So yes, some rumors are true: your... I mean your friend's... Mac can crash. No, probably not as often as your PC, but it happens.
If you do own a Mac, please check out Tom's other stuff on About.com: Macs. His site is next to none in his area of expertise and should be your first source for any Mac related tutorials and news.