it is helful to comment a list of all the colours at the top of the page, but i prefer doing this in the html right under the <html> tag. personal preference i suppose.
comments are helpful for certain rules that ensure compatibility among browsers and which browser they're for.
they might also be helpful for debugging, but there shouldn't be any debugging going since it's easier(for me at least) to just write the rules out again instead of commenting them out the whole time.
I think the advantages of comments should be fairly obvious. Even if something is very clear to you right now, it might not be so in a month's time, or not at all to somebody else.
I think both methods are useful. In the css style sheets while the code can show if the dolor is a basic one (e.g. #ccc or# ggg) or a shade or mix (e.g.
#D8DFE6) it is really important to know what basic color is being applied.(black, green , red, white, grey and so on). Comments in the html file however provide important information on structure.
It can however be really tempting to leave them out - to a designers cost at a later stage if he or she wants to change something.