Because sound waves do not produce color, it seems odd at first to characterize noise by color type. However, the characterizations do make sense. Sound colors are based on the way the frequency of a particular sound corresponds to the light wave frequencies of a particular color. The sound waves that make up pink noise, for example, have the same frequency as light waves that create the color pink.
The colors of noise are: white, pink, brown (red), blue (azure), purple (violet), and gray. White noise has a flat frequency in linear space; the corresponding light waves do not produce color. In addition, white noise is the only one that works in linear space. White noise is often referred to, in common use, as a pleasant background sound, such as waves crashing on a beach.
Log and Lin Space
However, not all sounds described as white noise actually produce a flat frequency. Unlike white noise, which works in linear space, pink noise works in logarithmic space. Like white noise, though, it has a flat frequency. But because pink noise works in a different type of space, it produces a different sound. In addition, due to it being a logarithmic sound, it has a proportionate bandwidth throughout each wave.
While blue (azure) noise, like the other noise colors, describes a specific frequency of sound, it has looser applications, similar to the way white noise does. When working with computer graphics, blue noise has come to mean any sort of low-frequency noise without any major spikes of energy.
Purple noise is much denser and more powerful than any of the other noises described so far, although it does not have any common usage the way blue and white noises do.
Gray noise works over a range of frequencies. It is also subjected to a psychoacoustic equal loudness curve. Both of these characteristics mean that gray noise sounds equally loud at all frequencies.
Like most of the other noises, brown (red) noise is logarithmic. However, brown (red) noise is a bit different in that it is not named for a particular color spectrum, but instead for a specific type of motion. Specifically, the name was adopted from Brownian motion, because brown noise is generated by the algorithm that produces Brownian motion. This is why brown noise can also be referred to as red noise. While it is named for Brownian motion, the waves correspond with red light waves.
While the above noises are considered the official noise colors, there are other colors used to describe sounds. The less-official colors are red, orange, green, and black. While red noise is typically synonymous with brown noise, it can also be a synonym for pink noise, as pink and red light wave have similar frequencies. This explains many of the similarities between pink and brown noise. Red noise works as another descriptor: it is used to characterize noise from different sources, termed oceanic ambient noise. The color red is applied to oceanic ambient noise because of the way the ocean absorbs high sound frequencies.
Green noise also fits several descriptions. It is often described as a long-term power spectrum averaged over several sites, and is considered the background noise of the world. When applied to this definition, green noise highly resembles pink noise. Green noise is also considered the mid-range frequency of white noise, or is thought of as bounded brown noise.
Orange noise, on the other hand, has only one description. It is a low-energy, finite, quasi-stationary sound. The frequencies orange noise produces are centered on the frequencies of musical notes in a scale. The sound generated by an out-of-tune ensemble is orange noise.
The last unofficial noise color, black, is an extremely interesting one. Black noise is also considered silent noise. Like many of the unofficial sound colors, it has several definitions. The most basic definition is to consider black noise as silence. Similarly, black noise describes what comes out of an active noise control system. While it is not silence itself, it cancels out the other noises in a space, therefore generating silence. Black noise is also said to describe noises with limited spectrums. Some researchers suggest that such low-frequency black noise may accompany natural disasters such as floods and power outages. Black noise is often also considered to be ultrasonic white noise.