Which tertiary colour can show both warm and cool influences?

Correct answer: Red-violet

The governing concept is that a tertiary colour combines neighbouring hues and may inherit their temperature qualities. Red-violet contains warm red and violet, which can include a cool blue influence, so its appearance may show both warm and cool character. Yellow-orange and red-orange are predominantly warm, while yellow-green is usually yellow-warm or green-cool rather than the intended red-and-cool-violet mixture. Thus A is correct.