This step by step tutorial will show you how to split tone with Color Balance in GIMP. Split tone photos are black and white photos that have been tinted with subtle color tones. Unlike more common methods of tinting, where a single color is applied to the whole photo, split toning applies a different color tint to the shadows and highlights which leads to a subtle visual color shift across the photo.
In the days of film photography, split toning was achieved by processing film with different chemicals to those specified by the manufacturer, which would lead to color shifts within the image. Certain combinations of films and chemicals would produce different results, but in the digital darkroom, you can achieve just about any color combination that you desire.
As with most things in GIMP, there's more than one way to produce this effect, but creating a split tone with Color Balance is perhaps the easiest way.
The first step will be to convert the original digital photo to black and white and again there are several ways to achieve this also. I'll quickly show you how to do it using the Channel Mixer feature and then once you've got a mono photo, applying the split tone with Color Balance will take no time at all.