Ben's got it pretty dead-on, most comics had a black key with around 3 colors (sometimes more, sometimes less), which could for a long time be chosen by the printer but eventually came to more standardized choices and eventually our good old cmyk. because halftone was harder to use than flat colors, many old comics and pulps preferred the latter. still, in any case, you can assume a black key followed by 3 color choices and possibly several colors achieved by a regular overprinting of 2 shades. this isn't likely with a lot of old books though because of the dominances, red and blue together made nothing but a very expensive black, while yellow in red made nothing but red and yellow in blue didn't make a very good color. by the 70's halftone and stuff had become far easier resulting in as many colors as you were willing to pay for.