You're right on the boost/setbacks. And I've acknowledged that Greens would be way to much fora voluntary Red. So I guess blues are what I'm looking for?
What I'm looking for is a way to reward players who want to do more than "I do the thing" and instead say "I do XYZ to give myself the risky edge".
Like in the latest Fast and Furious movie (I'll admit, the franchise is a guilty pleasure) a main character wins a street race by jury rigging a back ally nitro system to an old clunker. The engine blows out, lights on fire, it's totally insane, but it works.
I guess what I'm going for is what is the baseline response to "player voluntarily upgrades two Purples to Reds to try this stunt, what's the average reward they should get?"
Am I being too 'gamist' here? Is the answer always "well, it depends". I want to codify it so if I ever write a rules supplement hardliner GMs can't go "Lololol your five reds give you a single blue, thanks for being creative (not!)"
Ultimately it comes with the understanding that Dispairs are Really Bad ThingsTM, and not just Disadvantage+.