Positives were set up in copy cameras and four different negs made using color separation masks. One neg each for cyan, magenta, yellow, and black. The negs were then used to "burn" offset litho plates for four successive printing runs,with a fifth run to add text. At least thats how I remember the process from college(late 60's). Around 1970, or 71, printing firms started using "computers" to aid in the color separation process, as well as color mixing.BTW my step mothers family owned the largest printing facility in Michigan (Adair Publishing) and I've seen the procedure in practice with monster web presses --WOWWWW -- its a awesome experience!!
Rick