If a red and blue mixture looks muddy brown instead of violet, what is the most likely cause?

Correct answer: Yellow contamination

The governing concept is pigment mixing. Red and blue should produce a violet family colour when the pigments and tools are clean. Accidental yellow contamination introduces the third primary colour and can neutralise the mixture, making it dull or muddy brown. A shortage of either red or blue would mainly shift the hue toward the other component, and white paper does not explain brown pigment contamination.