A mixture contains red and yellow yet does not form a clean orange. What is the most likely cause?

Correct answer: One colour has a blue bias or contamination

The governing concept is pigment bias and subtractive mixing. A red or yellow pigment with a blue bias, or contamination from a previous colour, introduces an unwanted third-primary influence. Blue is complementary to orange, so it reduces saturation and makes the mixture muddy instead of clean orange; therefore, option A is correct. The fact that both pigments are warm is normally helpful, a white palette does not alter the mixture, and brush size does not determine pigment hue. Use clean tools and suitable pigments.