Even with a well calibrated monitor, it is more precise to color correct using numerical values. This is especially useful when correcting color casts in images with neutral gray areas, or when matching specific colors between multiple images.
Shift-click up to 4 areas of your image with Photoshop’s eyedropper tool to identify color sample points you want to watch in the Info palette (Option-Shift to remove them). You can also add color samples this way from within any color correction dialog box such as Curves, Levels, or Hue/Saturation.
Source: Source: Drawn from the “Color Correction by the Numbers” section in the book Photoshop CS3 Photographer’s Handbook co-written by Luminious Works trainer and founder Steve Laskevitch (read our announcement about our upcoming partnership with Luminious Works for Seattle-area Workshops).