Edges appear pink after applying bright red over a white base in poster colour. What is the likely cause?

Correct answer: The red layer being too thin at the edges

The governing concept is opacity and even coverage. If the red paint is spread too thinly at the edges, the white undercoat remains visible through it; red mixed visually with white appears pink. A dry white base is normally helpful, while pure red and thick paper do not explain the colour change. Apply an even, sufficiently opaque layer without repeatedly disturbing the edge.