Which colour is generally used to lighten poster colour?

Correct answer: White

The governing concept is value: adding white to a colour raises its lightness and creates a tint. Therefore, white is the correct choice. Black lowers value and creates a shade, while brown and violet change the hue rather than simply lightening it. White should be added gradually because too much can make the colour weak or chalky. This principle is useful when preparing light areas, highlights, or soft poster backgrounds.