What should be done if a colour mixture becomes darker than expected?

Correct answer: Add suitable white or a lighter base colour

The governing concept is corrective adjustment of colour value. If a mixture is too dark, adding a suitable amount of white raises its lightness; adding a lighter version of one base colour can also adjust the result while retaining a related hue. Therefore, option B is correct. More black would darken it further, a large amount of complementary colour could muddy it, and an unclean brush would introduce contamination.