While the three additive primaries of red, green, and blue combine into white light, mixing two of the additive primaries results in one of the subtractive primaries (for example, red + blue produces magenta). Combining two of the subtractive primaries results in one of the additive primaries (for example, cyan + yellow produces green).
A pure black ink is usually used with CMY for printing purposes. Mixing all three of the subtractive primaries results in a murky black-like color. Inkjet printer users may be familiar with this black. On some printers when the black ink cartridge runs out the printer will allow you to continue printing black by mixing the remaining CMY ink colors.