It is my understanding that a tertiary color is created when one mixes a primary and its complement, not the definition that you've given on your site. For example, a beautiful tertiary is BLUE + ORANGE, which is the same as saying BLUE + RED + YELLOW. In other words, the 3 primaries are present, thus the word 'tertiary'. I often come across this error, and don't understand why it's such a 'gray' area. Pun intended!

Great site - thank you!