From “Rainbows and Some Other Sky Phenomena”, by Roy Bishop
Reprinted in the 2013 Royal Astronomical Society of Canada Observer’s Handbook.

Your Rainbow:
A rainbow is located at a particular angle relative to the Sun-observer axis. When the observer moves, the axis moves, and the rainbow moves. The observer’s shadow moves too, but a rainbow is subtler than a shadow; you can see someone else’s shadow, but you cannot see their rainbow. Each observer has his or her own private rainbow. Although an observer’s eyes image different rainbows, the two rainbows are in the same parallel direction, so when the images fuse, the observer sees one rainbow at infinity. Obviously, a rainbow is not an object in the normal sense of the word; it is a pattern of light specific to the observer. Unlike the case of a normal object like a tree, there are as many rainbows as there are observers.

…

We Are Part of the Rainbow:
Although rain and sunlight have been part of Earth’s environment for four billion years, coloured rainbows occurred quite recently with the appearance of animals possessing colour vision. Our visual world with its brightness and colours occurs within our skull. Nevertheless, by some feat of mental projection we think that our visual world coincides spatially with the external world. Thus we confuse the neural rainbow with the external rainbow, and naively attribute the indescribable colours of the former to the latter. Three centuries ago, Sir Isaac Newton, aware of this overpowering illusion, wrote: “The rays to speak properly are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour.” Yet, even today, most people regard the cone cells of the retina as “colour receptors,” as if colours existed in the external world. They speak of “the true colours of moonbows and nebulae revealed by time-exposure photographs,” unaware that they are attributing a unique property of the neural photograph in their brain to the external photograph in their hands, and subsequently to moonbows and nebulae in the sky. The eye does not detect the colours of the rainbow; the brain creates them. We are part of the rainbow, its most beautiful part.