Which of the following hues is in harmony with each other?

The hues of red-violet, violet, and red are in harmony with each other because they are analogous colors on the color wheel. Analogous colors are groups of three colors that are next to each other on the wheel and often create serene and comfortable designs when used together. In this case, red-violet and violet share similar visual properties due to their proximity to each other, leading to a cohesive and aesthetically pleasing combination. The presence of red in this selection further enhances the harmony since it is adjacent to red-violet on the color wheel, creating a warm, pleasing palette.

The other groups presented do not exhibit the same level of color harmony. For instance, red, red-orange, and yellow contain adjacent colors but may not blend as seamlessly as red-violet, violet, and red. Similarly, blue, blue-violet, and red-violet create a mix of cool and warm colors that can clash, and green, orange, and red represent colors that are more dispersed across the wheel, leading to a less harmonious effect. Therefore, the selected answer showcases a more effective use of analogous colors.