Which of these is NOT a primary color?

The correct answer identifies that green is not a primary color in the traditional color model used in processes like painting or printing. In the context of additive color mixing (such as light), the primary colors are red, green, and blue. However, in subtractive color mixing, which is often used in art and printing, the primary colors are red, yellow, and blue.

Green is actually considered a secondary color in these models, created by mixing blue and yellow. Therefore, the identification of green as not being a primary color aligns correctly with the understanding of color mixing principles. Understanding these foundational concepts helps in recognizing the roles colors play in various applications of graphic communication.