A color wheel is easiest to understand when either a basic circle or six intersecting spokes serve as a framework for the colors. In the spokes arrangement, the lines are drawn 30 degrees apart. Attached to the ends of the spokes are 12 circles that hold color.
The primary colors are red, yellow and blue. They stand on their own and can't be created from other colors. Indeed, mixing them creates all other colors. On the spokes, yellow goes in the top circle with red to the left and blue to the right, four circles away from yellow. On the circle, three equal wedges of color show the primaries.
Three secondary colors are made by mixing equal amounts of two primaries. On the spoke circle between yellow and red, two circles away from each of them, sits orange, a mixture of yellow and red. Green sits between yellow and blue, created from them. Meanwhile, between red and blue is violet (purple). Likewise on color circle divided into six 60-degree wedges.
The six tertiary colors are created on the empty circles of the spoke wheel by equally combining the colors that sit on either side of each empty circle. The results are yellow-orange, yellow-green, blue-green, blue-violet, red-violet and red-orange. Adding the tertiaries to the circle, meanwhile, results in the final arrangement of 12 30-degree wedges.
The colors opposite on the wheel are complements of each other. Using two complimentary colors in unequal proportions brings out the colors better than if a color is used alone. Thus, a yellow flower with purple markings--say, a pansy--is more pleasing to look at than it would be with yellow alone.
Three colors sitting together on the wheel are analogous. Thus, blue-green, blue, and blue-violet are analogous and related in color. Together, they are harmonious, simplifying potential palette choices. A palette of three analogous colors plus the compliment of whichever color of the three will be most used will be pleasing to the eye.