What can be added if the blue of a watercolour sky is too vivid and needs to look natural?

Correct answer: A tiny amount of orange or grey

Colour intensity can be reduced by mixing a small quantity of a complementary colour or a neutral colour. Orange is complementary to blue, so a tiny amount partly neutralises the overly vivid blue and makes the sky appear softer and more natural. Grey can also mute it. More blue or red may strengthen the colour, while a large amount of black can make the wash heavy and muddy. Thus, option B is the suitable correction.