Why may mixing too much red and green in watercolour produce brown?

Correct answer: They are complementary colours and neutralise each other

The governing concept is complementary colour mixing. Red and green lie opposite each other on the colour wheel, so combining substantial amounts reduces their saturation and produces a muted neutral, often brown in watercolour. Water does not itself create black, and paper colour is not the main cause. Option B is correct because the complementary pigments partially neutralise one another; using less pigment or more water would produce a lighter, less muddy result.