Eliminating unwanted color casts (Photoshop)

If your scanned image contains an unwanted color cast, you can perform a simple test to determine whether the cast was introduced by your scanner. If it was, you can use the same test file to create a color-cast correction for all images scanned with the scanner.

To identify and correct a color cast introduced by a scanner:

1. Make sure that your monitor has been calibrated. 
2. Open a new Photoshop file, and use the Linear Gradient tool to create a blend from pure black to pure white.
3. Choose Image > Adjustments > Posterize, and posterize the blend using 11 levels.
4. Print the 11-step gray wedge on a black-and-white printer, and then scan it into Photoshop.
Note: You can also perform this test using an 18-percent neutral gray card or an 11-step gray wedge from a photography store.
5. Open the Info palette, and read the RGB values on-screen for each of the gray levels. Uneven R, G, and B values indicate a color cast.
6. Use Levels or Curves to correct the color cast, and then save the dialog box settings. (See 
7. Open the scanned image you want to correct, reopen the dialog box you used to correct the cast in step 6, and load the saved settings.