Which subtle colour mixture is most suitable for creating a warm grey?

Correct answer: Yellow-orange

The governing concept is colour temperature: a neutral grey becomes warm when it has a slight yellow or orange bias. Therefore, yellow-orange is the best choice because it adds warmth without removing the grey character. Blue-green, blue-violet, and pure blue introduce cool bias instead, so they would produce a cool grey or a distinctly cool colour rather than a warm neutral.