Purple is not in the visible light spectrum, but neither is red, orange, yellow or any other color. Colors are phenomenal sensations. They only exist in our brains.

The visible light spectrum is the region of the electromagnetic spectrum that human eyes see. It runs from wavelength of about 400 nanometers (nm) at the violet end of the spectrum to around 700 nm at the red end of the spectrum.

Why does adding red light with blue light give purple light? Our eyes have sensitive rod cells that roughly act as our black and white vision, as well as less sensitive cone cells which distinguish color. Three sets of cones exist, responding to red, green, and blue light.

Colors consisting of a single wavelength are called spectral colors or pure colors. The human eye and brain can distinguish many more colors than those of the spectrum. For example, purple and magenta are the brain's way of bridging the gap between red and violet.