Which colour can be added in a very small amount to give yellow a natural darker tone?

Correct answer: Violet

The governing concept is complementary colour mixing in the RYB system. Violet is opposite yellow on the traditional colour wheel, so a very small amount reduces yellow’s intense chroma and creates a deeper, more natural, slightly muted tone. The amount must be controlled because too much violet will neutralize the yellow and may produce brown or grey. White would lighten it, red would warm and shift it toward orange, and green would change the hue without being the direct complement.