Which mixture can create a dark yet relatively natural shade of red?

Correct answer: Add a little green to red

The governing concept is controlled use of a complementary colour to mute and deepen a hue. Green is complementary to red in the traditional RYB model, so a small amount can reduce red’s intensity and create a subdued, natural-looking dark red. The quantity must be limited: too much green will neutralize the red and push the result toward brown. White lightens, yellow shifts red toward orange, and water mainly changes transparency rather than creating a deep pigment shade.