Color theory encompasses a multitude of definitions, concepts and design applications. All the information would fill several encyclopedias. As an introduction, here are a few basic concepts.
The Color Wheel
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. Differences of opinion about the validity of one format over another continue to provoke debate. In reality, any color circle or color wheel which presents a logically arranged sequence of pure hues has merit.
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
Green, orange and purple
These are the colors formed by mixing the primary colors.
Yellow-orange, red-orange, red-purple, blue-purple, blue-green and yellow-green.
These are the colors formed by mixing one primary and one secondary color.