Mark (Photo_Utopia): very intrigued by your process. Will read up on it and may get back with some questions.
It's basically the same as making colour photos for tri-gum/gum-over-cyanotype tri-colour. I've experimented with it with lighting gels over a box brownie and got a similar effect with a cloud to Mark's shadow... and weirdly I can't find my photoshopped example anywhere
You put a Red gel/filter over the camera, calculate for proper exposure, take picture, then put a Green filter over it, calculate for proper exposure take picture, then put a Blue filter (a very dark blue one, hard to find) and recalculate proper exposure and take a picture. Then that translates to CMY (Cyan, Magenta, Yellow) So you print the Red as Cyan, the Green as Magenta and the Blue as Yellow.
Trust me, this does all work out apparently.
Most gum printers these days use digital negatives though. Doing colour separation is a bit easier that way...
And it's something I've wanted to do for a long time but I'm starting to think I don't have the patience for.