Grey noise

Grey noise is random noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies.

Grey noise spectrum

This is in contrast to white noise, noise which is in fact equally loud at all frequencies but not perceived as such due to psychoacoustics. See Colors of noise.

Note that humans perceive noise loudness based on amount of energy per octave, not amount of energy per hertz, so pink noise is closer to "equally loud at all frequencies" than white noise is. Similarly, a grey noise weighting curve must produce a psychoacoustically flat curve on a plot of energy per octave, not on a plot of energy per Hz (like an FFT). http://www.acousticfields.com/white-noise-definition-vs-pink-noise/