Which tertiary colour is useful in a deep evening sky?

Correct answer: Blue-violet

A deep evening sky is commonly shown with cool, dark blue and violet influences. Blue-violet is a tertiary colour that combines blue with violet, so it can communicate depth, coolness and the fading light of evening. Yellow-green, yellow-orange and red-orange are brighter or warmer choices and are less suitable for the stated atmosphere. Thus, option B is correct.