Which principle best explains why mixing yellow and blue may produce brownish colour instead of green?

Correct answer: Presence of opposing colour biases in the pigments

The governing principle is pigment bias and subtractive mixing. A blue pigment may lean toward violet, while a yellow pigment may lean toward orange; those hidden biases introduce a red component. The mixture then contains all three primary tendencies and becomes less saturated, often brownish, instead of a clean green. Paper reflection, green's value and brush size do not determine this hue result.