In traditional color theory, tertiary colors are browns and grays, containing all three primary colors. They're created by mixing either all three primary colors or a primary and secondary color (secondary colors of course being made from two primaries). By varying the proportions of each primary color, you create the different tertiary colors.
There are some schools of modern art that define tertiary colors as what you get when you mix a primary with an adjacent secondary color. As in when you mix a blue (primary) with a green (secondary). But, what you're getting is a shade of secondary with a stronger bias towards, for instance a bluer green or a yellower green, it's not the third level of color that the term "tertiary" implies. That occurs when you mix a primary with a non-adjacent secondary, i.e. all the primaries together. For instance, blue (primary) with orange (secondary), which is blue + red + yellow.
