You can use the eyedroppers in the Levels or Curves dialog boxes to correct a color cast—an unwanted tint from an excess of color (red, green, blue, or cyan, magenta, yellow). It’s easier to color-balance an image by first identifying an area that should be neutral and then removing the color cast from that area. Depending on the image, you may want to use one or all three of the eyedroppers. The eyedroppers work best on an image with easily identified neutrals.
For the best results, don’t use the eyedroppers in images that require a large adjustment to map a pixel to the maximum highlight or minimum shadow values.