John;

Cyan used to create a neutral density will cause a varying color bias as you adjust filtration due to the impurity in the filter coloration when compared to a true neutral. As an example, cyan adds a variable yellow component as you change the cyan filter level. This would require a reduction in yellow as cyan goes up, otherwise the beam begins to turn yellow instead of remaining neutral.

Cyan filtration should be confined to only reversal printing unless there is a gross error in the negative film processing or exposure conditions, or the film is bad with severe fogging. These are rare conditions such that if you meet with them, you have a problem with the system somewhere.

Average "GOOD" use should require M+C or Y+C for Ilfochrome and M+Y for all color negatives.

In over 50 years of color work, I have never had to do otherwise, nor have I known of anyone else who has unless there is a problem as I describe.

PE