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Matt Kloskowski has created a great video tutorial on using this aspect of the Adjustment Brush that I think is a must-see for any Lightroom 2 user.
A bonus tip is that you can also sample colors from the photo you are working on.
1. Click the color swatch to open the Color Picker.
2. Click into the color picker and hold as you drag the eye dropper out of the picker into the photo.
3. Release the mouse when you have chosen the color from the photo.
You should see the point in the picker change as you move over the photo. The color swatch will also change as you move around the photo. You can even tweak the Hue and Saturation in the color picker by pressing the arrow keys once you've selected the color.