They've added an 'oscillate' (fade up and down rhythmically, basically) option at SimplyNoise.com since I visited last. Very cool!
I like the pink noise setting on oscillate mode, and the brown even more. The pink is halfway between radio hiss and ocean, whereas the brown is mostly rainstorm, and a lot 'softer' on my ears than white noise. Unsurprising. I have a tendency to hate treble and cleave unto bass as one from a womb with a cardiac abutment untimely ripp'd. Bass nice.
The difference between brown and white noise, from what I gather from Wikipedia, is that with brown noise there is progressively less volume (amplitude) towards the higher end of the frequency range, whereas white noise has constant volume across all frequencies. Pink is halfway between them, its name explainable by the fact that brown noise is also called red noise. (Or Brownian noise, after the guy who discovered Brownian motion.)
That suggests you can't really have 'true' white noise, because it'd have to have the same power at ALL frequencies, which'd take an infinite amount of energy (except for absolute silence, I suppose?). The same goes for pink noise, because the power keeps decreasing but never quite reaches zero however high you go in frequency. However, it is possible to make pink or white noise across a limited range of frequencies, which, I presume, is what I'm listening to. My ears only have a limited range of frequencies*, so that's fine.
*20Hz–20kHz is the typical range of human hearing (optimal/for youngsters).
Am I — blast it all, I'm trying to educate again. TELL ME WHEN I START DOING THAT.