Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 136

Figure 2.61
colorcast.png after
automatic tonality
correction has been
applied (prior to cor-
recting the red color
channel)
Now you should see the histogram for the red color channel in the
Levels window. Underneath the histogram are the adjustable triangles that
you are already familiar with. You can skip the black triangle for shadows and
the white triangle for highlights—I'd say that the automatic correction did a
perfect job balancing them.
Click and hold the left mouse button on the gray triangle and drag it to
the right. Doing this will change the mid-tones of the red color channel . If
you check the Preview option in the Levels window, you can immediately view
the adjusted image.
What next? Simply adjust the color according to your personal preference,
and voila! Save the image as colorbalance1.png in your exercise folder, and
you're done.
Another method for correcting images with color cast is by using the
black and white eyedroppers in the Levels window. You can click either of
these buttons to select a black or white spot, respectively.
If you use this second option, your image must contain true black or
white areas. Select the black eyedropper (tool tip: Pick black point ) and click
on the black area of the image. Then proceed to use the white eyedropper
(tool tip: Pick white point ) on the white area. This procedure is often sufficient
to correct the color levels.