In spite of sophisticated exposure-control systems, pictures taken with digital cameras often come out over- or under-exposed, or with color casts due to imperfections in lighting. Gimp gives you a variety of tools to correct colors in an image, ranging to automated tools that run with a simple button-click to highly sophisticated tools that give you many parameters of control. We will start with the simplest first.

Gimp gives you five automated color correction tools. Unfortunately they don't usually give you quite the results you are looking for, but they only take a moment to try out, and if nothing else they often give you an idea of some of the possibilities inherent in the image. Except for "Auto Levels", you can find them in the Layer menu, by following the menu path Layer->Colors->Auto in the image menu.

Here they are, with a few words about each:

If you can find a point in the image that ought to be perfect white, and a second point that ought to be perfect black, then you can use the Levels tool to do a semi-automatic adjustment that will often do a good job of fixing both brightness and colors throughout the image. First, bring up the Levels tool as previously described. Now, look down near the bottom of the Layers dialog for three buttons with symbols on them that look like eye-droppers (at least, that is what they are supposed to look like). The one on the left, if you mouse over it, shows its function to be "Pick Black Point". Click on this, then click on a point in the image that ought to be black---really truly perfectly black, not just sort of dark---and watch the image change. Next, click on the rightmost of the three buttons ("Pick White Point"), and then click a point in the image that ought to be white, and once more watch the image change. If you are happy with the result, click the Okay button; otherwise Cancel.

In addition to "Multiply" and "Divide", you may every so often get useful effects with other layer combination modes, such as "Dodge", "Burn", or "Soft Light". It is all too easy, though, once you start playing with these things, to look away from the computer for a moment and suddenly find that you have just spent an hour twiddling parameters. Be warned: the more options you have, the harder it is to make a decision.