Theoretically black is not needed. However, when full-saturation cyan, magenta, and yellow inks are mixed equally on paper, the result is usually a dark brown, rather than black. Therefore, black ink is overprinted in darker areas to give a better appearance. Figure 3-4 shows how additive colors expressed in RGB space and subtractive colors expressed in CMYK space mix to form other colors. The ColorSync Manager performs those calculations for you when converting amon g color spaces.
Some color spaces can express color in a device-independent way.
Whereas RGB colors vary with display and scanner characteristics, and CMYK colors vary with printer, ink, and paper characteristics, device-independent colors are meant to be true representations of colors as perceived by the human eye. The most common method of identifying color within a color space is a three-dimensional geometry. The three color attributes, hue, saturation, and brightness, are measured, assigned numeric values, and plotted within the color space. RGB colors vary with display characteristics, and CMYK colors vary with printer, ink, and paper characteristics. Conversion from an RGB color space to a CMYK color space involves a number of variables.