This script converts the color temperature of an image. You can specify the color temperature of the original image, and the desired color temperature of the result, as well as a brightness correction factor. The script installs itself under the menu Filters/Colors/Convert Color Temperature.
The full documentation for the script is available at http://luca.dealfaro.org/Colortemp
The script works for Gimp 2.2, 2.4, and 2.6 .