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Eyedroppers, found in the Levels and Curves dialog boxes, can also be used to restore dynamic range and remove color casts. It's usually easier to use the eyedroppers in the Levels dialog box as you can Alt-drag the input sliders to help determine black/white points. The following eyedroppers are available:
Black eyedropper. Click on the black eyedropper and select a pixel that you want to be black in the image. From the Levels dialog box, Alt-drag the black input slider to help identify dark shadows in the image.
White eyedropper. Click on the white eyedropper and select a pixel that you want to be white in the image. From the Levels dialog box, Alt-drag the white input slider to help identify bright highlights in the image.
Gray eyedropper. Click on the gray eyedropper and select a pixel that you want to be gray. This eyedropper removes color casts in midtones. It's not looking for an exact gray (128,128,128), but an initial value of 92 to 160 for each channel is reasonable. It then balances the three RGB components so they have the same value (a shade of gray), and proportionally applies the correction to the rest of the image.
These adjustments are illustrated in the following figure. While the black/white eyedroppers do a credible job of re-establishing dynamic range, it is difficult to remove color casts in this image with the gray eyedropper. There were no obvious gray values in the image!