What is the best way to distinguish the three planes of a cube in watercolour?

Correct answer: Use three distinct values according to the light

To show the three planes of a cube, the artist must first establish one light direction and then assign values according to how each plane faces that light. Three related but distinct light, middle, and dark values clarify the planes and their spatial orientation, so option B is correct. Equal values flatten the cube, black removes useful variation, and changing only the outline cannot describe surface direction.