Taking a closer look at the color wheel, you can see a triangle made up of the three primary colors -- red, blue, and yellow. They are called primary because they can't be made by mixing other colors together.
Between the primary colors on the wheel are secondary colors, which are made by mixing two primaries:
orange = red + yellow
green = yellow + blue
violet = blue + red
Going one step farther, there are tertiary colors, such as blue-green and yellow-orange, between the primaries and secondaries, and an infinite spectrum of colors between those. In our gardens, we find more of these types of colors than the "pure" primary and secondary colors. (Artists also use the term tertiary colors to mean colors created by mixing all three primary colors to produce many of the colors found in nature from mustard yellow, browns -- from yellowish to reddish to deep umber -- and finally black.)
Image: Courtesy Don Jusko
A full color wheel that shows more of these gradations is a closer representation of the spectrum of colors than the simple one we’re using to understand the basic concepts.