Go to Cookbook

Color Balance...

This opens a similar window as for "Brightness/Contrast".

You select a given color, and then play with minimum, maximum values and global brightness 

If you apply this to a grey image, the modifications are identical to "Brightness/Contrast", regardless of the color you select

If you apply this to a color image (RGB for exemple), the modifications are applied to the selected color only. Thus, you may remove the Red part of the image, or increase the brightness of the Green part only. If you modify a color that is not encoded in the image, nothing occurs.