Jeffrey and Matt are correct this is additive mixing of blue and green light (through dichroic filters) rather than subtractive mixing. The Beseler 45 Universal VC head worked the same way.

The key for OP to get started is to know that variable contrast papers bascially have two emulsions, a low contrast layer sensitive to green light, and a high contrast layer sensitive to blue light. By varying the proportions of green and blue light reaching the paper you change the contrast. More green relative to blue = lower contrast. More blue relative to green = higher contrast.

Roughly, if you dial all the way to green you get grade 00 or 0, and all the way to blue/violet would be maximum contrast (anywhere between grade 4 and 5+). A middle setting giving approximately equal amounts of blue and green light would be around grade 2 to 2.5.