In the traditional RYB colour wheel, why is red mixed with green not classified as a standard tertiary colour?

Correct answer: They are complementary colours, not a neighbouring primary-secondary pair

In the traditional RYB colour-wheel convention, a standard tertiary colour is named for a neighbouring primary and secondary pair, such as red-orange. Red and green are opposite, complementary colours on that wheel, so their mixture does not fit that naming category. Mixing their pigments may produce a muted colour, depending on the pigments, but that does not make it a standard wheel tertiary.