Jagr_68 posted...
Rotoscope?
Someone give the answer
"White screen" backlit using sodium vapor lights which emit only an extremely specific wavelength. You can then filter the sodium vapor light or filter everything EXCEPT the sodium vapor light to create the mask. Then you can delete the mask from the composite and splice in the color filter and you get near perfect transparency, color addition, and have access to a much wider range of colors.
The reason it was "lost" is because it required custom cameras with a custom splitter prism behind the lens to record the film into two reels at once, one with the filter and one with the mask. There was no way to just "buy more"
A guy managed to recreate the prism but not the camera so he just used two cameras instead.