What does webdesign have to do with graph paper? When I was still in webdesign school (for lack of a better word), graph paper didn't factor into it.Plus when you're working on a webdesign project, it helps to scratch out a rough idea of placement and sizes, plus keep hex-codes neat and clean for easy reference.
For that matter, what do hex-codes have to do with graph paper? They're nothing more then simple base-16 numbers; no real equations there. Plus the fact that you can't do math with hex-numbers intended as colors on a webpage -- every color is perceived differently by the colors surrounding it; there's more intuition then calculus going on there.