A color circle, based on red, yellow and blue, is traditional in the field of art. Sir Isaac Newton developed the first circular diagram of colors in 1666. Since then scientists and artists have studied and designed numerous variations of this concept.

PRIMARY COLORS
Red, yellow and blue In traditional color theory, these are the 3 pigment colors that can not be mixed or formed by any combination of other colors. All other colors are derived from these 3 hues

SECONDARY COLORS
Green, orange and purple
These are the colors formed by mixing the primary colors.

TERTIARY COLORS
Yellow-orange, red-orange, red-purple, blue-purple, blue-green and yellow-green.
These are the colors formed by mixing a primary and a secondary color.

COMPLEMENTARY COLORS

Complementary colors are any two colors which are directly opposite each other, such as red and green and red-purple and yellow-green. In the illustration above, there are several variations of yellow-green in the leaves and several variations of red-purple in the orchid. These opposing colors create maximum contrast and maximum stability.

 

COLOR HARMONY
In visual experiences, harmony is something that is pleasing to the eye. It engages the viewer and it creates an inner sense of order, a balance in the visual experience. When something is not harmonious, it's either boring or chaotic. At one extreme is a visual experience that is so bland that the viewer is not engaged. The human brain will reject under-stimulating information. At the other extreme is a visual experience that is so overdone, so chaotic that the viewer can't stand to look at it. The human brain rejects what it can not organize, what it can not understand. The visual task requires that we present a logical structure. Color harmony delivers visual interest and a sense of order.
In summary, extreme unity leads to under-stimulation, extreme complexity leads to over-stimulation. Harmony is a dynamic equilibrium.

Color Context
How color behaves in relation to other colors and shapes is a complex area of color theory. Compare the contrast effects of different color backgrounds for the same red square.

Nature provides a perfect departure point for color harmony. In the illustration above, red yellow and green and blue create a harmonious design, regardless of whether this combination fits into a technical formula for color harmony.

When choosing colors for your design, remember a few rules for mixing colors. The human eye cannot focus on red and blue at the same time. Trying to read red type on a blue background or vice versa causes extreme eye fatigue. Your audience will not be receptive to your message if it hurts them to read it. Never, ever use blue type on a red background and even worse, is red type on a blue background.
Most colors go well together with members of the same "family". Warm colors of type, such as red, brown, orange and yellow look better together in combination warm colored backgrounds; cool colored type like blue, green, gray and white with cool colored backgrounds. Using color families generally makes for a more appealing presentation, especially for large amounts of information.
Contrast is fun and can be used effectively to accent information and draw attention to items. In general, keep the contrast low. Too much contrast makes your work difficult to digest. For type, keep the contrast reasonable no matter what colors you’re working with. If your background has a value of, say 20%, and the type has a value of 80%, it presents a subtle look that's easy on the eyes and is graphically appealing. Avoid the extremes.

Red appears more brilliant against a black background and somewhat duller against the white background. In contrast with orange, the red appears lifeless; in contrast with blue-green, it exhibits brilliance. Notice that the red square appears larger on black than on other background colors.