The hex numbers used to represent colors in CSS are 3 pairs corresponding to Red Green and Blue. For example:

#0f100e

would be the same as RGB(15,16,14). Hex is used because the colors are calculated at 8 bits per colour and using hex makes the math a bit simpler than going from 00 to ff, rather than from 0 to 256. If you'd rather stick to good ol' base 10, CSS has a function rgb(rrr,ggg,bbb) where rrr, ggg, bbb represents a number from 0 to 256. You can also use percentage values as arguments too, so long as you specify the values with the % type modifier.

Two decent color charts are here:

http://devguru.com/technologies/css2/8023.asp

although color charts like this are all over the web. Here are some other decent intros to color:

http://www.j-a-b.net/web/hue/color-names
- has a "decoder ring" for decimal to hex :-)

http://en.wikipedia.org/wiki/Web_colors
- ever-venerable wikipedia

And because I'm a web-color idiot, here are some helper sites I've found useful:

http://www.colorschemer.com/online.html
- I like this one, give a nice coordinated palette of colours

http://www.wellstyled.com/tools/colorscheme2/index-en.html
- another scheme generator

http://www.colormatch.dk/
- and another

Yep. That too. :D