What green noise is
Green noise is an informal name, popularized online, for noise gathered in the middle of the range of hearing. Unlike white, pink and brown noise, it has no single official definition, so recordings labeled green noise can sound quite different.
SleepHz's Green Noise is centered on 500 Hz and fades gently above and below it. That takes away both the sharp hiss and the heavy rumble, leaving a calm, airy sound.
Why green noise sounds like nature
Many outdoor sounds, like wind in leaves, a distant river or surf heard from far away, carry most of their energy in the middle of the range. Green noise imitates that balance, which is why people describe it as the most natural-sounding noise color.
Green noise vs pink noise
Pink noise covers the full range of hearing, so it masks more kinds of sounds. Green noise drops the very low and very high pitches, so it's gentler but masks less. If you need to cover traffic, choose pink or brown noise; if you just want a calm background, try green.
Is there research on green noise?
Not yet. Green noise hasn't been studied on its own, so any benefit is personal preference. The noise research that does exist is mostly on pink noise and sleep and white noise for babies.