One example of a situation where it may be necessary to adjusting color balance is when photographs are taken under artificial light made using film designed for daylight. Artificial light, especially from incandescent tungsten light bulbs, is often rather yellow. (This is because the white-hot tungsten filament is much cooler, at about 3000K, than the surface of the sun, at about 5000K). Note that digital cameras usually have an 'automatic white balance' feature that almost eliminates this problem, but the automatic white balance can itself cause false color rendition if the picture includes large areas of the same color. Red sunsets for example, might look too pale because the automatic white balance always tries to eliminate any overall color bias in the picture. The following photo was taken in the Paris metro under fluorescent lighting which often gives photos a green cast.

To adjust the color balance, choose Layer>Colors>Color Balance. Make sure that preview, midtones and conserve luminosity are all selected, then slide the color sliders until the metro station has a gray tint rather than green or brown (think of brown as dark red when you are making this adjustment). I find that it is usually not necessary to change the balance for highlights or shadows, only for midtones. Save your picture once you have done your best to correct the color balance, and remember that the GIMP cannot do miracles and that sometimes the result will be less than ideal. A photograph taken underwater without flash, for example, is always likely to be lacking in red, even after color adjustment, since it is likely that all the red light will have been absorbed by the water.

You may be interested to know that the GIMP can attempt to automatically balance colors as well as adjust brightness and contrast - choose Layer>Colors>Auto>Equalize and check out the result. Sometimes the result is good, but in this case I find the result too blue and too harsh. If you are a perfectionist you will probably NOT want to use the automated adjustments.