What is the student's mistake if they try to make red-orange by mixing red and green?

Correct answer: Red-orange requires red and orange

The governing concept is that a tertiary colour is named from the two neighbouring colours used in its mixture. Red-orange is made by combining red with orange, so using red and green shows a wrong choice of component colours. Blue and green would produce blue-green, not red-orange; white alone cannot create this tertiary hue, and therefore saying there is no mistake is incorrect.