https://www.theatlantic.com/science/archive/2016/02/white-noise-sound-colors/462972/?single_page=true, posted Nov '18 by peter in audio cognition reference
Most people are familiar with white noise, that static sound of an air conditioner that lulls us to sleep by drowning out any background noise.
Except technically, the whirl of a fan or hum of the AC isn’t white noise at all. Many of the sounds we associate with white noise are actually pink noise, or brown, or green, or blue. In audio engineering, there’s a whole rainbow of noise colors, each with its own unique properties, that are used to produce music, help relaxation, and describe natural rhythms like the human heartbeat. If you know what to look for, you can start to notice the colors of the noise that make up the soundscape around us.