Which mixture can create a warm grey?

Correct answer: Add a little yellow-orange to grey

The governing concept is the warm or cool bias of a neutral colour. A small amount of yellow-orange introduces a warm hue while allowing grey to remain the dominant neutral colour, producing warm grey. More blue would make cool grey, and blue mixed with white produces a light blue tint. Green added to black gives a dark greenish mixture, not a clear warm grey. Hence, option D is the best answer.