An object appears white because all portions of white light illuminating it are reflected back to the viewer.
In theory, an overprint of all three process colors absorbs all portions of white light and produces the appearance of black. In reality, a muddy brown is produced. For this reason, black is used as a fourth printing ink.
The illustrations above show process inks printed on white paper. Each process printing ink (cyan, magenta, yellow) absorbs or subtracts certain portions of white light and reflects other portions back to the viewer. Process printing inks are transparent. It is the paper that reflects unabsorbed light back to the viewer.
