With Illustrator CS3, Adobe added a TON of functionality around the use of color. Initially referred to as "Live Color", these features included the ability to get inspired around the use of color (the Color Guide panel, Adobe Kuler integration, etc.), and the ability to change or edit colors (Recolor Artwork, etc.).
With Illustrator CS4, Adobe enhanced some of these features to make them a bit easier to use and understand (more consistent terminology, contextual menu support, etc.). Most importantly, in my opinion anyway, Adobe dropped the "Live Color" marketing term and clearly identifies features in the program (for example, the Recolor Artwork feature opens a dialog called Recolor Artwork instead of Live Color).
Admittedly, the Recolor Artwork dialog features a plethora of settings - which gives you an idea of just how powerful the feature actually is - but it can be hard to figure out how to use it to do just what you want (or need) it to do. Hopefully, I can help you out with that...
I wrote an article for iStockPhoto called Recoloring Artwork, where I take you step-by-step through the process of changing specific colors in a piece of artwork. The article shows how to change the color of a woman's hair and eyes -- even though the colors are employed inside of gradients. Along the way, you'll learn how to better organize the colors in your document, as well as learn how to take advantage of the power found in the Recolor Artwork dialog. An added bonus is a quick way to instantly convert color artwork to grayscale in Illustrator. Enjoy!