Which of the following is classified as a primary color?

Blue is classified as a primary color because it cannot be created by mixing other colors together. In color theory, primary colors are the foundation of the color wheel, from which all other colors can be derived by mixing. The primary colors—red, blue, and yellow (or sometimes considered as cyan, magenta, and yellow in subtractive color theory)—are fundamental in creating a wide spectrum of hues.

The other colors listed—orange, violet, and green—are secondary or tertiary colors, formed by mixing primary colors. For example, orange is created by mixing red and yellow, violet by mixing blue and red, and green by mixing blue and yellow. These color mixtures rely on the existence of the primary colors, further emphasizing the foundational role that blue plays in the color system.