Remove Color Cast: Levels & Curves Tools

This tutorial is done in GIMP 2.6.1. If you are using a different version you may face several differences in menu locations or tools panels.

If you don't know what a color cast means please check Wikipedia or any source you want. Basically it means that the photography presents an unwanted unbalance between RBG channels (red, green and blue colors).

There are more possibilities to correct color cast but I will present here just my favorite methods, using Levels and Curves tools.

The first method, using levels, works only in case you have a photo with neutral gray points (neutral mid-tones). A perfect neutral gray point is a pixel having 50% values on each color channel.

The first part presented will be common to both approaches. This is the original image (I don not remember the link, but it's freely available from http://gimp-savvy.com):

Most of the images from this tutorial can be enlarged by cicking on them.

Step 1: Duplicate The Layer

Wow, we have some blue in this photo isn't it? In order to perform color cast corrections we need 3 points from our image: black, white and a neutral. With the second method (curves tool) you can use more points in order to fine tune the final result.

For this photo it is relatively easy to detect 3 reasonable points, but I what to use a way which will help you also with more complicated situations and which does not count on the way your monitor displays the colors. In order to do this we will use the threshold tool.

We will duplicate the background layer, using the Layer menu (Layer / Duplicate Layer), right clicking on the layer in Layers dialog and selecting Duplicate Layer, clicking the Duplicate Layer button on the layers dialog or using the shortcut: Shift + Ctrl + D.