Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 139

Within the Color Balance window, you will see three sliders set at the zero
position. Their purpose is to adjust the selected range's color levels, and there
are three buttons, each specifying a range to be modified: Shadows , Midtones ,
and Highlights .
You can move the sliders to one side to increase the color level of a certain
color in the image, or you move them toward the complementary color—i.e.,
red to cyan, green to magenta, or blue to yellow—to balance out the color.
You can preselect one of three areas of brightness where the adjustments are
to be applied.
Uncheck the Preserve luminosity check box, but leave the Preview option
checked.
This function is appropriate for minor color corrections or to intensify
colors.
The following values will yield a good result when applied to the example
image:

• NOTE
The Channel Mixer is yet another
function of GIMP that can be used
for color balancing. What's more,
you can use this tool to create fancy
or even psychedelic atmospheres
for your image. Find this tool by
choosing Colors > Components >
Channel Mixer . The Channel Mixer
has a greater importance in editing
black-and-white photography. I will
go into more detail in section 4.1.3.

Shadow Red

Green
Blue

0
0
-50

Midtones Red

Green
Blue

-66
50
100

Highlights Red

Green
Blue

-66
0
50

Of course, you can change the other colors and adjust the image to your
taste. Feel free to experiment!
If you are in the mood, you can subsequently perform a tonality correction or
post-edit the brightness and contrast using the Brightness-Contrast function.
Save the finished image as colorbalance2.png in your exercise folder.