How are gradual darker versions of the same poster colour created?

Correct answer: By gradually increasing a very small amount of black

The governing concept is a shade scale: a base colour becomes progressively darker when very small, controlled amounts of black are added. The artist should mix gradually and compare each sample, because excess black can overpower the hue and make the result dull. Increasing white creates lighter tints, while adding more water changes consistency and may reduce coverage rather than reliably darken the colour. Equal complementary colour can neutralise or muddy the mixture, so option B is correct.