What is a common way to lighten an acrylic colour?

Correct answer: Add white colour

The governing concept is colour value and tinting. Adding white to an acrylic colour raises its value and creates a tint, making the colour appear lighter while retaining a related hue. Black does the opposite by creating a shade, and a complementary colour generally dulls or neutralises the mixture rather than simply lightening it. Varnish may change surface sheen or transparency but is not the standard colour-lightening method. Therefore option D is correct.