What is a likely reason for obtaining brown instead of a clean orange?

Correct answer: A small amount of blue entered the mixture

The governing concept is complementary-colour neutralisation. Orange is the secondary colour produced by mixing red and yellow. Blue is opposite orange on the colour wheel, so even a small amount of blue can reduce orange’s saturation and shift the mixture toward brown or grey. Therefore option B is correct. Clean red and yellow, equal quantities, or a new brush do not normally create brown; contamination or dirty tools are the more plausible explanation.