Its been a LONG time since I looked into it, but if I recall correctly they were listed as:

25% Magenta=R(red) 2

50%Magenta=R3

100% Magenta=R

And so on for Cyan (B-blue) and Yellow (Y)

So the YBR2 of Xavier's jacket would be 100%Yellow, 100%Cyan, 25% Magenta.

The guides were given to the printers who would cut out overlays of each of these, which were then turned into the printing plates.

As the better papers and printing technology became available, more percentages of colors became available, as well as the early fully painted comics. The old fully painted comics were done on bluelines, a process where the B/W art was transferred to illustration or watercolor board as a blue line, literally. A mylar or acetate overlay of the B/W art was also created for reference. The B/W art became the black plate and the color art was shot and separated into the CMY plates. In some early full color books or graphic novels, the blueline under the B/W art can sometimes be seen if the registration is off....

I've simplified the process some and I'm typing from 20-25 year old memories, but that's a pretty close description of the process.