This plugin changes the color temperature of an image via shifting it in the YUV color space:
http://en.wikipedia.org/wiki/YUV

Compile and install it with:

 $ CC="g++ -O2" gimptool-2.0 --install elsamuko-temperature.c

Menu entry is Colors -> Color temperature.

Windows binaries from François or Sylvie:
http://registry.gimp.org/files/elsamuko-temperature_w32_inst.zip
or
http://www.aljacom.com/~gimp/elsamuko-temperature-32bits-64bits.zip

Mac binaries (for the macports GIMP):
http://sites.google.com/site/elsamuko/gimp/temperature/elsamuko-temperat...

Example of a warm setting with lowered exposure:



The YUV color space with the shift, the plugin does:
(cc) Tonyle/Wikipedia

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