Why does adding black not automatically make a warm colour cool?

Correct answer: Black mainly darkens value

The governing concept is the difference between value and colour temperature. Mixing black generally creates a shade by lowering the value, or making the colour darker. The original hue bias can remain warm even when its value changes, although particular pigments may behave differently. Black is not automatically a complementary colour and does not always eliminate hue. Therefore, option A correctly identifies the principal effect and explains why darkness alone does not mean coolness.