I’ve been working an an auto white balance tool in GMIC and decided to put it to the test with this image. Turns out it works really well. Here is the result with the input as linear tiff exported out of Rawdigger, imported into GIMP and in the GMIC plug-in. No other processing.
The filter works by trying to maximize the variation in hues so you don’t need any neutral grey areas at all. In fact, neutral grey would not work for this method and often you don’t need to adjust anything.
I’ve put the filter in y Testing → Iain Fergusson in the GMIC plugin if you want to test it out.