Three separate images, comprising a red, green and blue layer, are displayed on the screen rapidly, one after the other, relying on persistence of vision effects to merge the three layers into a single coloured image.
This effect is simulated in another image linked to here (warning: the linked image flashes colors very rapidly, which could potentially cause seizures in people with photosensitive epilepsy); the flashing is less noticeable when viewed on a television screen.