To start, just click anywhere in the colored area on the right or on one of the Basic Colors on the left. You can then do a fine adjustment with the slider arrow on the right.
IE Users can move the slider on the far right by left-clicking and holding down the mouse button while you slide the arrow. Other browser users can simply click above or below the to adjust.
If you like a color, click "Add Custom" to add it to the grid of Custom Colors. You can return to any of those color values by clicking on the Custom Color square.