Which of the following is NOT considered a primary color of vision?

In the context of vision, the primary colors are those that can be combined in various ways to create a broad spectrum of colors. In human vision, the primary colors are red, green, and blue. This is based on the trichromatic theory of color vision, which suggests that there are three types of color receptors (cones) in the retina that are sensitive to different wavelengths of light corresponding to red, blue, and green.

Yellow, while a visible color, is not classified as a primary color in this model. Instead, it can be created by combining red and green light. Hence, it is considered a secondary color in terms of additive color mixing, as it results from the combination of two of the primary colors. The distinction between primary and secondary colors in the context of vision is fundamental to understanding how we perceive color and how colors interact with one another.