As I recall most color negative are not simply negative but have some orange mask to take into calculation.
Not sure what this entails, but when I did my lot of negs, I used CaptureNX2 to process them.
CNX2 didn't have any 'one click' method to reverse negatives, but someone commented that reversing the curve may do the trick.
In CNX2 it did .. perfectly(if exposure was within a decent range), and from that point I saved in CNX2 as a batch process, and applied to all neg to NEF captures.
ViewNX2 didn't have the ability to do tone curves, but it seemingly did via the Picture Control add on.
But what that did, was simply reverse the brightness/tone curve, and leave the colour channels as is. So your darks would turn bright, but your greens wouldn't turn red, or blues turn yellow .. etc.
An interesting Picture Control setting, but definitely not colour reversal. So CNX2 was my only way forward from there.
Never touched any colour masks .. orange or any other colour.
So if the D850 has this one click colour reversal feature, then obviously it can be done in camera.
My assumption when trying to use the Picture Control software was that the camera simply couldn't do the colour reversal.
D850 obviously can, but I would have preferred to see it done as a Picture Control variable.