I use to do some spelunking and on many occasions lost light while underground (usually failed batteries, one time my light took a bad hit. Thankfully I always had backups). I wouldn't really describe the color as "gray" though, there is too much randomly colored static for that. More of a very dark soupy brown. Sort of like construction paper perhaps.
Slight correction: the colour of rushing water's noise is not white. More like Brownian (http://en.wikipedia.org/wiki/Brownian_noise). White noise is rather jarring on the ear.
This is a total aside, but I've always wondered: is the ringing a repeating waveform? If you could discover its structure through trial and error (e.g, cycling through pure tones and looking for beat frequencies), could you create an anti-ringing real waveform that nullifies the tinnitus? In other words make a hearing aide + your ear physiology act like a noise cancelling headphone, at least as far as your auditory nerves are concerned.
My limited understanding is that it is from one of the hairs in your inner ear being bent or broken in some way - the hair itself doesn't move, but since it's bent in the way airwaves could are able to bend it momentarily, the brain interprets it as it being in that momentary state permanently. Waveform cancellation happens at a physical level - it wouldn't "unbend" the hair as the hair isn't actually moving.
I don't actually know though, this is just my understanding.
