What is a subtle way to break the uniformity of blue-violet in a night scene?

Correct answer: Add small red-violet and muted warm shifts

The governing concept is controlled colour variation within a unified palette. Red-violet is relatively warmer than blue-violet, so a few small red-violet notes or muted warm shifts can create visual variety and guide the eye without destroying the nocturnal mood. Option A is correct because the change is limited and harmonious. Vivid yellow across the whole scene would overpower the palette, black would remove colour information, and white outlines would create a harsh graphic effect rather than subtle variation.