The horseshoe-shaped diagram at the right shows the CIE 1931 chromaticity diagram, a mathematical illustration of color space. The spectrum locus, as it is sometimes called, shows the boundaries of visual color space. In other words, the area inside this locus represents every color that the normal human eye can see. Note that the three dimensions of color are flattened into a single two-dimensional plane which ignores the lightness of a color, and therefore, this diagram should not be equated with the appearance of a color.
RGB: RGB, or red, green and blue, are the additive primaries that are the basic elements of white light. By mixing amounts of RGB, other colors are made. Because they are spectrally pure primaries, RGB provides a very wide range of colors. The downside of the RGB color space is that most of the colors in this gamut cannot be displayed on standard computer monitors and cannot be printed.
sRGB: The sRGB, or “standard” RGB, color space was developed by Microsoft and Hewlett-Packard as a device-independent color space that is compatible with most computer monitors and other hardware. Its features include a D65 (6500 K) white point, a monitor gamma of 2.2 and various other standards for viewing. Its intent is to provide uniform viewing conditions across various monitors resulting in consistent visual results.
CMYK: CMYK (the color space enclosed by the magenta line in the diagram at the right) is a subtractive color model used in color printing. This color model is based on mixing pigments of C=Cyan, M=Magenta, Y=Yellow and K=Key (black) to mix other colors.
CMYK is in some respects the opposite of RGB color space, as it is based on using colored ink to progressively obscure an already white background. The cyan, magenta and yellow colors may be thought of as alternative primary colors to red, green and blue. In theory, equal quantities of CMY should produce black, but the use of the fourth “color” black may be more reliable.