Why is the sky blue?
Combine all colors in light and you get white or combine any three colors equidistant on the color wheel to produce white light, usually red + green + blue producing magenta, turquoise, and yellow shadows.
To see a color the wavelength must be in the light source. When you see color, you are seeing wavelengths reflected back to you, the rest of the colors are being absorbed by the surface.
Why isn't brown in the spectrum? Because brown is the combination of many colors both in light and paint.
How we measure color in light? A part of the Electromagnetic Spectrum called Visible Light is measured in nanometers, nm. This information is useful when understanding the color rendering ability and color bias of various light sources when compared to daylight.
The primary colors in light; red + blue-violet + green combine to make white light. The shadows in pure color light make the complimentary colors, found opposite on the color wheel, magenta, turquoise, yellow.
Sunset colors are particles in the atmosphere capturing and reflecting light.
To understand why this is so, one need only recall how typical sky colors are produced. The familiar blue of the daytime sky is the result of the selective scattering of sunlight by air molecules. Scattering is the scientific term used to describe the reflection or re-direction of light by small particles .... At sunrise or sunset, sunlight takes a much longer path through the atmosphere than during the middle part of the day. Because this lengthened path results in an increased amount of violet and blue light being scattered out of the beam by the nearly infinite number of scattering "events" that occur along the way (a process collectively known as multiple scattering), the light that reaches an observer early or late in the day is noticeably reddened. Thus, it could be said that sunsets are red because the daytime sky is blue. 
If you are looking for a rainbow, turn your back to the sun at the end of a rainstorm. Particles in the sky, droplets of moisture, split the sunlight in the same way a prism will, creating distinct bands of color. Each water droplet acts like a tiny prism. Light is refracted upon entering the droplet, then reflected onto the back of the droplet, then refracted a second time upon exiting the droplet.
A double rainbow forms when light is reflected twice on the inside of the droplet.
Color Vision in Insects and animals
Animals have vision that differs from humans, bees and butterflies see colors we cannot see, more Ultra Violet wavelengths, guiding them to the patterns in flowers. Dogs and cats mostly see."... greys and some blues and yellows." 
"... In general, humans can see wavelengths of the electromagnetic spectrum from 400 to 800 nanometers - from violet to red. Insects, on the other hand, perceive wavelengths of from 650 to 300 nanometers, including the ultraviolet range of the spectrum. "  Circadian Animals
Electromagnetic Spectrum & Visible Light
Visible light is only part of the Electromagnetic Spectrum surrounding us, X rays;, Ultra Violet Light, UV, Visible Light, Infa Red Light, Microwaves, Radio Waves, and Long waves.
390 to 700 Nanometers, nm, describes the wavelengths visible to the human eye.