Color printing yellow (minus-blue) overprinted by magenta (minus green) yields red. Where all three inks may overlap, almost all incident light is absorbed or subtracted, yielding near black, but in practical terms it is better and cheaper to use a separate black ink instead of combining three colored inks. The secondary or subtractive colors cyan, magenta and yellow may be considered "primary" by printers and watercolorists (whose basic inks and paints are transparent). Two graphic techniques are required to prepare images for four-color printing. In the "pre-press" stage, original images are translated into forms that can be used on a printing press,
C-41 process not is a subject of some debate. C-41 film consists of an acetate or polyester film base, onto which multiple emulsions are coated. Each layer is only sensitive to a certain color of visible light. In the classic illustrative example, there are three emulsions: one is red sensitive, another is green sensitive, and the top is blue-sensitive. Beneath the blue layer is a yellow filter, composed of dyes or colloidal silver. All silver-based photographic emulsions have some sensitivity to blue light, regardless of what other colors they may be sensitized for. This filter layer serves to remove the blue light,