Which tertiary colour may be suitable for showing depth in a night sky?

Correct answer: Blue-violet

The governing concept is the use of colour temperature and value to create visual depth. Blue-violet is a cool tertiary colour made by combining blue and violet, and its cool, dark-looking character can suggest a distant, calm, deep night sky. Yellow-orange and red-orange are warm and visually advancing, while yellow-green is brighter and less associated with deep night space. Thus, option A is the most suitable answer.