Try making a +3000 increase in Temperature using GIMP’s color temperature operation. First try starting from a temperature of 2000 and increasing to 5000. The try starting from 8000 and increasing to 11000. I see quite a difference. Do you?
Are you sure? My old Canon camera does have color temperature information that can be seen using “exiftool /path/to/file”, for example:
WB RGGB Levels As Shot : 730 1096 1096 1027
Color Temp As Shot : 2950
WB RGGB Levels Auto : 2515 1024 1024 1646
Color Temp Auto : 4984
WB RGGB Levels Measured : 2512 1018 1029 1645
Color Temp Measured : 4984
Of course auto temp is easily fooled. And the temeprature as shot might not have any relationship to the actual temperature of the ambient light.
The thing is, GIMP’s code for Color Temperature is a lookup table derived from, well, I’m not exactly sure what the original calculations were based on. That table is very old, though results are pleasing.
The right way to code a color temperature tool is to do a Bradford chromatic adaptation on the colors in the image, from a presumed starting temperature to the desired destination temperature. Even this sort of operation leaves to one side “color of the light” considerations that move along the magenta-green axis.
IMHO the best use for GIMP’s Color Temperature operation is “move the sliders until you like the look” and forget about “are these really the right colors”.
If you don’t like results from GIMP, I’d suggest using RawTherapee, darktable, PhotoFlow, etc, all of which have green-magenta sliders as well as color temperature sliders. But it really depends on your goal. And changing the presumed starting temp does affect the output.
If you try all these softwares, please share your results here!
Are you trying to white balance an improperly white-balanced image? For the “top of the line” approach for this situation, if none of GIMP/darktable/PhotoFlow/RawTherapee color temperature sliders work for you, and doing a normal white balance by clicking on a nominally neutral “gray patch” isn’t working for you, you might try RawTherapee’s CIECAM module.
Or maybe you are going for an artistic change in the overall image “color of the light”? All the tools will work, but again RawTherapee’s CIECAM module is excellent, though the learning curve is a bit steep and the full array of stuff to try is overwhelming.