Screen printing, also referred to as silk screening, is the process of transferring an image to a printed surface by ink, which is pressed through a stenciled screen and treated with a light sensitive emulsion. Film positives are put in contact with the screens and exposed to light, hardening the emulsion not protected by the film leaving a soft area on the screen for the ink to pass through, with the help of a squeegee.
Each screen is only good for one color. You must create multiple screens in order to accommodate the multiple colors in an image. Each color goes on separately and requires drying time between colors.