What is the main difference between adding black and adding blue-green to red-orange?

Correct answer: Black darkens value while blue-green reduces saturation

The governing concept is the difference between shading and complementary desaturation. Black lowers the value of red-orange, producing a darker shade. Blue-green is its complementary tertiary partner; adding it mainly reduces saturation and makes the red-orange duller, although pigment mixtures can also affect value. Option A states the intended primary distinction. Neither additive makes the colour lighter, primary, or correctly described by calling black complementary and blue-green neutral.