During 2e, lots of products had two colors, mostly cyan/black (the core books come to mind) or magenta/black (the 2e Draconomicon).
Later on, they experimented with black + special color (technical bable: you replace the ink drums in your printer with an ink drum filled with a custom-color, like the brownish red found in, say, Complete Book of Elves). Every now and then you'd also have a 4-color page, like the gorgeous John & Laura Lakey picture that opened up the 2e DMG (the one with the gorgeous blonde holding a mirror that reflects an ogre).
The most expensive line TSR had was Planescape. Not only were all pages 4-color (thanks to the borders), they also seemed to have a special color (the muted gold highlights), making it a 5-color page (and yes, there are colors you can't achieve with CMYK, like acid greens, metallic colors, sharp aqua blues and stuff).