If equal amounts of red and yellow pigments produce a muddy orange, what is the most likely cause?

Correct answer: Contamination by a complementary colour

In subtractive pigment mixing, red and yellow should normally produce an orange hue. A small amount of an unintended complementary pigment can absorb additional wavelengths and lower the mixture’s saturation, making the orange appear dull or muddy. Brightness alone does not introduce a neutralising colour, temperature is not the relevant variable here, and paper thickness does not change the pigment mixture itself. Therefore A is correct.