If equal red and yellow produce a duller orange than expected, what is the most suitable conclusion?

Correct answer: A component may have a blue or green bias or contamination

The governing concept is pigment purity and colour bias. Equal proportions describe quantity, but they cannot remove an unwanted blue or green bias in either paint; contamination from an unclean brush or palette can also reduce orange intensity. Equal mixing is not automatically wrong, and white is not required to create orange. Therefore option D gives the most reasonable diagnosis.