If orange made from red and yellow appears muddier than expected what is the most likely reason?

Correct answer: Unwanted blue contamination

The governing colour-theory concept is complementary neutralisation. Blue is opposite orange on the traditional colour wheel, so even a small amount of blue contamination can absorb or counter part of the orange mixture and lower its saturation, making it look dull or muddy. Therefore option B is correct. A clean brush prevents contamination, suitable yellow supports the mixture, and white paper generally affects appearance rather than causing muddy pigment.