Which of the following colors is known as a tertiary color?

The correct choice, yellow-green, is indeed recognized as a tertiary color within the color wheel. Tertiary colors are created by mixing a primary color with a secondary color. In this case, yellow, which is a primary color, is combined with green, a secondary color that results from mixing the primary colors blue and yellow.

By blending these two, yellow-green emerges as a unique hue that sits between yellow and green on the color wheel, demonstrating the characteristics of both the primary and secondary colors involved in its creation. The understanding of color relationships is essential in art, as it helps artists create depth, mood, and visual interest in their work.

The other options presented, such as red, blue-orange, and purple, do not meet the criteria for tertiary colors. Red is a primary color, and purple is a secondary color made from mixing blue and red. Blue-orange, while it may seem descriptive, is not a standard designation for a recognized color within the context of primary and secondary combinations. Thus, yellow-green is the only choice that correctly identifies a tertiary color.