Which color is NOT considered a secondary color?

In color theory, secondary colors are created by mixing equal parts of two primary colors. The primary colors are red, blue, and yellow. When these primary colors mix, they produce secondary colors: red and yellow create orange, red and blue create purple, and yellow and blue create green.

Given this understanding, red is classified as a primary color, not a secondary color. It stands on its own in the color wheel and cannot be formed by mixing other colors. Therefore, red is the only color listed that does not fall under the category of secondary colors, as orange, purple, and green are all formed by combining primary colors.