What is the main difference between adding grey and adding black to a colour?

Correct answer: Grey creates a tone while black creates a shade

The governing concept is the distinction between tone and shade. Mixing grey into a hue creates a tone, usually reducing its intensity while modifying its value. Mixing black creates a shade and generally moves the colour toward a darker value. The effects are not merely changes of name, are not always identical, and do not define transparency. Therefore, option D is correct.