Rob's Color Picker blog entry at Macworld was interesting, but IMO, he missed out on the best Color Picker hint...

Go to Image Palettes (where the Spectrum usually appears; it's the next-to-last icon in the toolbar), and you can paste a new graphic in there, or choose an image file. Just use the Palette pop-up menu below the color area, and choose either New from File or (with something on the clipboard) New from Pasteboard.

The Color Picker remembers these images globally, so you can always have a reference picture from which to pick colours.