Toward which colour will blue-violet shift if blue is reduced and violet is increased?

Correct answer: Toward violet

The governing concept is relative component proportion in a tertiary colour. Blue-violet is made from blue and violet. Reducing blue while increasing violet makes violet the more influential component, so the sample shifts toward violet. It would not move toward green or yellow because those are not the components named in the mixture. A redward shift would require a stronger red influence, not merely more violet.