For me, it's easier to inspect the element in Firebug and copy/search the hex code, since I get the guarantee that I'm not changing an unrelated poorly named variable.

I find that for simpler sites, naming colors after their closest tone works well, but when I have a dozen shades of the same color, I start to run out of names very quickly (e.g. it's not unusual for me for a single page to have different background boxes, zebra striped tables, subheaders, selected/hover states and an array of other details on top of the overall layout)