We just finished doing a quick & dirty test of the Kolari Pro 3-Stop ND screw-on filter. Screenshot #1 was auto white balanced with no filter; #2 is the same white balance, only with the Kolari ND3 filter; #3 was white balanced with the ND3 filter in place. While all land directly on the skin tone line, there are appreciable color differences between the three clips and the Kolari cannot be called neutral by any stretch of the imagination. Video coming soon.
The Daejeon Chronicles 
We can conclude that none of the brand that announce their filters to be neutral are fair.
However, it would be worse with variable ND, at least I was never satisfied by them, even if it’s very practical.