The CSS3 Color Module much more solid than the other CSS3 proposals and experimental features. In fact, as of June 2011, it’s an official W3C Recommendation. New CSS3 Color features include HSL color specifications in addition to traditional hex and RGB values, as well as the ability to use opacity and gradients.
As a primer for using these new color features, I wrote an article over at The Code Project called CSS3 for Beginners: Color and Opacity. This should get you started. If you find the article helpful, please vote it up and share your thoughts in the article comments.