Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 135

2.6.3 Using the Levels Function
to Correct Color Cast
Open the image colorcast.png in the SampleImages folder on the topic's DVD
and save it in your exercise folder.
You will notice that the image has a distinct red color cast, which means
that the red color channel values are faulty.
Figure 2.60
The colorcast.png image has a strong red color cast.
You can use the function Levels in the Colors > Levels menu to correct
the bright-dark values in the image. Using the Levels dialog that appears, first
change the settings of the RGB color channel by clicking the Auto button.
The image should be visibly improved, as now the real black and white
tones can be seen. For some images with a slight color cast, this action may
be sufficient.
By clicking the Channel drop-down menu on the top in the Levels window,
you can choose the red, green, and blue color channels separately so that
you can correct each color individually. Select the red color channel.