Managing and editing colors is central for the digital photography workflow. White balance is one of the cornerstones of color editing. Setting the “right” white balance is most often a question of subjective judgment, interpretation of the scene and its different light sources, or even plain, simple aesthetics.
Digital cameras have controls to make that judgment, automatically, by way of fixed setting, or by precise measurement of the light, usually when pointed at a gray card. And then they have RAW file, where the values are passed as-is, unbalanced, for further processing.
So when qtpfsgui or any other HDR programs merge a few RAWs into an HDR image, no color editing is done yet. Nor is it done during tonemapping. So the resulting tonemapped image often has a color cast, like the one below:
The next step is to open it in an image editor and do some color work. In many RAW converters, including proprietary Adobe Lightroom and open source RAWstudio there is a very convenient tool: click on a single point in the image that you think should have neutral (gray) tone. It will be sampled and color will be automatically adjusted accordingly. Despite a very complete set of color tools I did not find anything like this in the GIMP. I found a script by professor Luca de Alfaro to add this functionality. As a script it is slower than a native tool, and it takes more steps (specifically: one extra action in the menu), but it works.