Hi gang - I'm a newbie, trying to figure out how things work

I was playing with the software on the OpenSourceCMS site, trying to get a feel for it before I commit to building my site with it... I noticed that in the CSS sheets, all the colors are defined by three digit codes... I was wondering what those were, because in my experience, HTML uses 6 digit codes to define colors... when I tried changing the "practice" css sheet, I didn't see any changes, and I wondered if this was the reason why... I don't want to use "word" definitions, like "red" because I'm looking to get a bit more nuanced in my color selections....

Thanks!