A color separation is produced by taking a photograph of the original art using a specific-color filter on the lens and black and white transparency film in the camera. Further, for printing purposes, a halftone-screen mask was placed over the art so that the film camera captured dots instead of continuous tone.
The color filters are Red, Green, and Blue and negatives of these shots represent Cyan Magenta and Yellow.
There were 4 plates (for a short time there were 3 colors CMY, but a 4th was added).
A full plate's worth of imagery and type was on a large transparency placed in contact with the (photosensitive) plate surface and then placed in front of a high-intensity arc light.
The transparency itself was "pasted up" by hand from photographically produced type and random images (sometimes called randoms).
The plate then has the appropriate ink applied.
Even in the digital age, circa 1989, I used a black and white video camera with a manual rotating color wheel attached to an Amiga computer to digitize color images for animation.
If I remember correctly, you can manually convert and RGB digital image to CMY by converting the RGB values to a percentage and then subtracting that from 100%.