Which principle is most useful for naturally blending light and dark areas in facial skin?

Correct answer: Use subtle warm and cool changes within the local colour

Natural skin blending depends on gradual value changes together with subtle colour-temperature shifts. Warm and cool variations within the local skin colour can describe light, reflected colour and form without making shadows dead. Adding only black often dulls the mixture, hard edges interrupt transitions, and one uniform colour removes modelling. Therefore option B gives the most convincing approach.