Which colour should be increased to make red-violet more violet?

Correct answer: Violet

The governing concept is that increasing one component of a mixture shifts the mixture toward that component. Red-violet is made from red and violet. Increasing violet strengthens the violet influence and makes the colour appear more violet. Yellow, green, and orange are not the two components named in red-violet; adding them would change the hue in another direction instead of producing the intended result.