Which of the following colors is a tertiary color?

A tertiary color is created by mixing a primary color with a secondary color. In the color wheel, primary colors include red, blue, and yellow, while secondary colors are formed by combining two primary colors; for example, mixing red and blue creates purple, yellow and blue create green, and red and yellow make orange.

Red-orange, as the correct answer, is a tertiary color because it is formed by mixing the primary color red with the secondary color orange. The addition of orange, which itself is created by mixing red and yellow, results in the specific hue known as red-orange. This color is often used in design for its vibrancy and ability to complement various color schemes.

The other colors listed—red, blue, and yellow—are all primary colors and do not fit the definition of tertiary colors since they haven't been combined with secondary colors to form a new hue.