Color mixing by a halftoning process, as used for color reproduction in graphic arts and many forms of digital hardcopy, is neither additive nor subtractive. Halftone color reproduction with a given set of primary colors is heavily influenced not only by the colorimetric properties of the full-tone primaries, but also by effects such as optical and physical dot gain and the halftone geometry. By computer simulations based on basic spectrophotometric measurements on actual prints, we demonstrate that such effects not only distort the transfer characteristics of the process, but also have an impact on the size of the color gamut. In particular, a large dot gain, which is commonly regarded as an unwanted distortion, expands the color gamut quite considerably. We also present an image processing model that can describe and quantify the effects of physical and optical dot gain on different media and with different halftoning methods.