Well, if you consider overall "greenness" you also have to consider the impacts of manufacturing the components.
We're all going to print on paper, and there's not that much difference between COT320 or Arches Platine and copier paper to make quick proofs in an ink jet, in terms of effluents from the process. So, we can pretty much leave the paper out of the equation, unless you know someone who makes his own from post-consumer recycled cotton/linen rags.
That said, potassium dichromate requires mining and smelting chromium ores, as well as purifying some potassium compound (likely potassium hydroxide). Chromium is nasty, nasty stuff, but we're using tiny quantities of it -- and Richard Sullivan was recently talking about an alternate, organic sensitizer for his precoated carbon tissues, that might or might not be superior to dichromate either in use or in manufacture.
For cyanotype, we have ferric ammonium citrate, created from food by-products and iron (microscopic amounts of iron relative to industrial steel production), and potassium ferricyanide, drawing on organics, iron (again) and potassium as above. No terribly toxic metals, no carcinogens. Trace amounts of potassium dichromate are used to add contrast where needed, but if negatives are created specifically for cyanotype dichromate isn't likely to be needed, and for photogrammetry you'll generally accept the contrast you find.
So, for my money, gum, carbon, and similar dichromate/colloid processes are just about in a toss-up with cyanotype. Given I can make dextrin, a form of gum, at home by cooking starch, and will need a small amount of potassium dichromate for any of these, I'm inclined to suggest that for my use gum bichromate is the greenest -- less transportation cost/fuel consumption, a minimum of dedicated chemical production, and even the pigments can be very green -- with cyanotype a close second; carbon is third only because, though I can get gelatin in the supermarket that will work, it's known as a "cranky" process that will tend to produce a lot of waste.
All, of course, should be exposed with sunlight, not artificial UV...
