A fine white line turns grey when painted over a dark poster colour base. What is the likely cause?

Correct answer: Dark-colour residue remaining in the brush used for white paint

The governing concept is accidental colour contamination during opaque painting. If a brush still contains even a small amount of dark pigment, that residue mixes into the white paint and turns the fine line grey. A completely dry dark base normally helps a clean opaque highlight remain distinct, while a thin brush is appropriate for detail and thick paper does not by itself change white paint to grey. Washing and drying the brush before loading white prevents the problem.