The biggest challenge is just keeping the tones consistent across the entire film, while at the same time avoiding the ‘samey’ look of an overagressive blanket color grade. It’s a fine line, and from the example frames above it looks like this monotone look could be a problem when using this software. With the 3 frames that Dre chose, you can see the variation in the skin tones from location to location. Yet with the two examples, the color of Obi-wan’s face looks the same in the desert as it does in the Death Star.
In reality you would use at least 10 reference frames in varying conditions to train the algorithm. So, if properly trained, the algorithm should not have a problem with skin tones under varrying lighting conditions.