I don't even really know how old-school coloring worked, although I'd be interested (from an academic perspective) to find out.
The colorist would color a reduced photocopy of the inks with Dr. Martin Dyes (similiar to watercolors), and then code each color they used. This would be used as a guide for the color separators/printers (Do a Google Image Search on "Comic book color guide" to get an idea of what these looked like).
The separator would then physically cut the plate to be used for each color in the CMYK 4-color printing process (Cyan, Magenta, Yellow, Black). Those plate would then be used in the printing of the actual book. That was the process I originally learned at the Joe Kubert School back in '94, and it was already on its way out by then.
The process had its limitations, which is why primary colors were so prominently used in the first few decades of comics. The Hulk was colored grey in his first issue, but they switched his coloring to green because of their difficulties in maintaining a consistent grey tone.