There’s an interesting discussion over at Diglloyd about color shifts when stopping down the Sigma Art 105mm f/1.4 (though this shift can be seen to some degree in other lenses as well).
When it comes to custom white balance, Lloyd Chambers has this to say:
Then there is the question of correction for secondary color, which itself can impart a significant greenish cast to background out of focus areas, and magenta cast to OOF foreground areas (which is why a gray card must always be shot in focus and never even a little out of focus).
I also noticed a green tinge in the OOF background of a few of my videos shot with the GH5 and white balanced with an X-Rite Colorchecker (which I never bothered to focus). Apparently some brands and lenses are more susceptible to this effect than others. Still, I’m not entirely ruling out the possibility that the beta LUTs I was testing might have contributed to the green tinge.
Something else to test in another life!