The problem is in changing the RGB values so that the white balance stays the same, as the system is not linear. I think this is best done in HSV space. A code snipped for doing pixel value conversion using qimageblitz (http://api.kde.org/kdesupport-api/kd...ageblitz/html/)
Qt Code: 
  1. unsigned int *data, *end;
  2.     data = (unsigned int *)qimage->bits();
  3.     end = data + (qim->width() * qim->height());
  4.     InlineHSV hsv;
  5.     while(data < end){
  6.         // convert pixel to HSV
  7.         hsv.convertRGB2HSV(BlitzPrivate::convertFromPremult(*data));
  8.         // change H, S or V as needed
  9.         hsv.setSaturation(somethingBasedOn(hsv.saturation());
  10.         // and back to RGB
  11.         hsv.convertHSV2RGB();
  12.         (*data) = BlitzPrivate::convertToPremult(qRgba(hsv.red(), hsv.green(),
  13.                                                      hsv.blue(), qAlpha(*data)));
  14.         data++;
  15.     }
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