Pure yellow poster colour looks dull over a dry dark layer. What is the most effective correction?

Correct answer: Apply an opaque white base before adding yellow

The governing concept is opacity and underpainting. A dark dried layer absorbs or visually overpowers a bright yellow, reducing its apparent saturation. An opaque white base blocks that dark underlayer and provides a light ground from which yellow can retain its brightness; therefore option B is correct. More water makes coverage weaker, while black or a darker underlayer makes the yellow duller. Let the white base dry before applying yellow.