In my Star Wars project I was able to use some of the color (chroma) of 35mm sources overlaid on the blu-ray, but this requires almost frame by frame image registration to prevent misalignment due to the natural gate weave of the film. I would not recommend mixing and matching luma between sources because humans can much more easily detect luma than they can chroma, and the alignment would have to be perfect.

The channel mixer is a powerful tool, and I used that in conjunction with a curves adjustment to remove the green tint in the Blu-ray: (from the Matrix 53mm thread) http://screenshotcomparison.com/comparison/209768

My guidelines for channel mixer use are to leave the ‘constant’ values at 0, and make sure that each of the 3 channels equals 100.