Why is the outer color of a rainbow red?
Rainbows are created under certain light conditions when water droplets in the atmosphere act like a prism and refract sunlight.

Light is part of the electromagnetic spectrum which covers electromagnetic waves from radio waves with a wavelength of 1000 m (long wavelengths) to gamma rays with wavelengths around 0.01 nm (short wavelengths).

Sunlight is composed largely of all the colors of the spectrum of visible light. When our eyes see all the colors at once, our brain interprets the color as being "white light".

When humans see a red object that object reflects red light by absorbs all the other colors of light.

In the spectrum of visible light, red light has the longest wavelength of 700 nm and violet light has the shortest wavelength of 400 nm.

When light enters a water droplet in the atmosphere the water droplet acts like a mini prism. The angle at which the different wavelengths of light exit the prism is determined by Snell's Law and this is why light that exist a prism, or is in a rainbow, is layered into colors with red being the first layer and violet the last.