Contamination by which colour may make an orange mixture appear brownish?

Correct answer: Blue

Orange is a secondary colour made by mixing red and yellow. On the traditional colour wheel, blue is opposite orange and is therefore its complementary colour. If blue accidentally enters the mixture, it partially neutralises the orange, reducing its brightness and making it appear dull, brownish, or muddy. Yellow and red are ingredients of orange, and white lightens it rather than causing this neutralisation. Thus, option B is correct.