Okay, now I am confused. Les yousay yellow doesn't work to soften contrast but then you say you use a 70 yellow???? Anyway, I have found all of the old contract filters that were used to assist in halftoning and the "add contrast" filters are varying densities of magenta and the "lower contrast" filters are all shades of straw or yellow if you will.
But, what I don't get is the whole split filtering concept, here. Are you applying a filter then shading out the parts that you don't want effected by that filter, applying another filter and exposing the parts you were just shading?

As for theCMY vs RGB, cyan opposes Red, yellow opposes blue, and magenta opposes green. The problem with trying to discuss these things is. . . What color is yellow to you may not be the right yellow as far as photography is concerned. I have been fighting with this concept since I have been trying to incorporate color into the shop without a dichroic head. I have reverted to terms like cherry and salmon rather than red, because although they are both red; cherry brings up green in negative printing, while salmon brings up cyan. Straw yellow brings up a colbalt blue, while lemon yellow brings up more of a lavendar or indigo blue.

Anyway, I would like to understand this whole split contrast filtering concept. AND I wonder is it applicable to color or just b&w. I have been trying to find a way to adjust contrast in color, but so far I only adjust the color, not the contrast. Or with the neutral density filters, I simply change the exposure time but the image remains about the same in the end.