When is a lighter related colour better than white for lightening poster colour?

Correct answer: When a more saturated light result is needed

The governing concept is the relationship between value and saturation. Adding white usually raises the value but can reduce the original colour’s intensity, making it paler or chalkier. A lighter colour from the same family can raise the value while preserving more chroma and hue identity. Therefore option A is correct. Grey would neutralise the colour, a darkening mixture moves in the opposite direction, and erasing a background is unrelated to colour mixing.