A student mixed equal red and yellow but obtained a dull orange. What is the most suitable conclusion?

Correct answer: One colour may contain a blue or green contaminating bias

The governing concept is pigment purity and unwanted complementary bias. Red and yellow should normally produce orange, but a red or yellow containing a slight blue or green tendency can introduce a complementary component. That component reduces saturation, making the orange dull even when the proportions are equal. Therefore D is correct. Equal proportions are not automatically wrong, orange need not be premixed, and white is not required to create orange.