A primary color chart displays the primary colors red, blue, and yellow and their possible combinations. The chart usually takes the form of a Venn diagram with three overlapping circles, colored red, blue and yellow. The areas where they overlap give secondary colors.
Red, yellow, and blue are called primary colors because they are the only colors that cannot be obtained by mixing other colors together, and also because an artist can produce the widest variety of other colors by various mixtures of these three. This is noticeable in most printers' ink packages, which include yellow, magenta (red), and cyan (blue) ink to produce all the other colors.
A primary color chart almost always includes the three secondary colors, the "children" of the primary colors: orange (yellow mixed with red), violet/purple (red mixed with blue) and green (yellow mixed with blue). These colors fill the overlapping sections of the Venn diagram.
Finally, a basic color chart also includes the tertiary colors. Various combinations of the secondary colors with the primary colors produce tertiary colors. The tertiary colors on the chart are yellow-orange, red-orange, red-violet, blue-violet, blue-green and yellow-green. In total, a color chart gives 12 basic colors from where you can obtain further shades.