When doing color correction or trying to apply the zone system and adjust
exposure latitude it is handy to be able to set several reference points.
The color picker tool could be enhanced to allow you to set a reference or
anchor point. The point should be numbered and the values for the point
displayed - either on the image itself (perhaps with a transparent
background) or in a seperate docked window or with a float-over popup. The
reference point should have the ability to "snap-to-point" so if you find a
good middle grey for example, you can easily apply the
Levels-Grey-Eyedropper to the previously referenced point. Using the color
picker and adding the XY coordinates would be useful (ala Info-Window).
Showing the before and after pixel values would also be useful - imagine a
color-blind person being able to do color correction based on the pixel values.