In the traditional RYB colour wheel, why is a mixture of orange and violet not classified as a standard tertiary colour?

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

In the traditional RYB convention, a standard tertiary colour comes from a neighbouring primary and secondary hue, such as red and orange. Orange and violet are both secondary hues, so their mixture does not meet that naming convention. Their mixture may still be useful in painting; it is simply not a standard primary-secondary tertiary pair.