Just in case this is where the confusion arises...

The OP appears (from his comments) to have a light source that approaches variable contrast in a relatively rare way - it emits light in varying amounts of green and blue, and adjustments are done by adding different amounts of green and blue.

Most of the information around, including information concerning the sets of filters, deals with light sources that are relatively full spectrum, and filters that subtract differing amounts of green and blue light.

It is important for the OP to think of light as a mixture of red light, green light and blue light. The red light is visible to us, but doesn't affect the paper. If the mixture of the rest of the light has more blue light than green, the print will be higher in contrast. If the mixture of the rest of the light has more green light than blue, the print will be lower in contrast. So what we need to do is adjust the contrast by adjusting the ratio of blue to green in the light.

So when he/she reads about using yellow filters, he is reading about taking light which includes lots of red, green and blue and subtracting a portion of blue (to increase the relative amount of green, and therefore reduce contrast).

And when he/she reads about using magenta filters, he is reading about taking light which includes lots of red, green and blue and subtracting a portion of green (to increase the relative amount of blue, and therefore increase contrast).

The OP's variable contrast head works more directly.