It almost never happens in real life so normal photographers don’t have to deal with it, but if there is no green color at all (e.g., you mix magenta and blue s

It almost never happens in real life so normal photographers don’t have to deal with it, but if there is no green color at all (e.g., you mix magenta and blue so your green pixels are all at noise floor) then you cannot bring it back from nothing as you color-balance raw footage. You would have to do some coloring. However, human perception is such that color is much more subjective than things like artefacts from green screen separation, so it might actually not be a big deal.