Color correction is one of the most important, and most misunderstood, aspect of photo-editing in Adobe Photoshop. This article explaining color correction with the Curves eyedropper tool is based on an except from "Photoshop Basic for Photographers" a video course by photographer Phil Steele found at SteeleTraining.com.

In the Curves dialog, three eyedroppers appear below the histogram. These droppers are used to set the Black Point, Gray Point, and White Point, respectively.

To set the black point, find the darkest part of your photo, an area that should be true black. Select the Black Point dropper by clicking on it, and then click it once in the area of your photo that should be black. This re-maps the the tonal value of the image so that all points of that value or darker are now black.

To set the white point, follow the same process, but pick an area of the image that should be white. When you click on that area with the White Point Dropper, you remap the tones in your image so that all pixels of that value or lighter are white.

All midtones in the image are remapped to be proportionally distributed between these new black and white points. You have now corrected the tonal distribution but not the color. The Gray dropper does that.

The Gray dropper is trickier to use. In this case, you want to select the dropper and then pick on a point in the image that should be Neutral Gray. This does not mean a gray halfway between black and white (this is a common misunderstanding). It could be a middle gray, or it could be a very dark gray or a very light gray. It could even be almost black or almost white. What's important is that it is a gray that is "neutral" in color, not tinted with any color. For example, concrete in a photo often makes a useful neutral gray point.

If you photo has a color cast, the gray area may appear slightly tinted in the photo, but what you are doing by clicking on it is telling Photoshop "this area should not be tinted, it should be neutral gray." When you click it, the color cast should disappear and the photo should be color corrected.

If you try and fail to get a neutral gray point, you may get color results that look slightly off, or even significantly off. In that case, you may have to do some trial-and-error hunting, picking on gray points until you get one that seems to generate the desired results.

If you are really struggling to find a neutral gray point, you might try looking at the Info panel as you move the Gray Dropper over the image, and look for points where the RGB values are nearly equal. That indicates a point that is almost neutral gray, and clicking it may correct the color satisfactorily.

For a video version of this tutorial visit this YouTube link:
http://www.youtube.com/watch?v=QmX2xu73tW0