What colour is that noise?
No, this isn't about synesthesia, that cross-over effect in the senses where letters in the alphabet can have colours and sounds can smell — if you're like Nabokov or Duke Ellington or Ludwig Wittgenstein. (Okay, okay: no one is like Ludwig Wittgenstein.)
It's about noise, pure and not-so-simple, noise without tune or harmony but noise blended in just a certain way.
Let's start with white noise. Most people have heard of white noise and may even have used it to block out other and distracting sounds. Turns out, though, that white noise isn't just any boring blare that lets you disregard it through habituation. A true definition of white noise would take me into mathematics, where I dare not go. However, let's say that white noise has a "flat power spectral density," which seems to mean that at any frequency the signal contains equal power, much in the same way that a white light distributes power over the visual frequencies in a way that stimulates our colour receptors equally.
But hearing is seeing what I mean. So listen now to a 10 second patch of white noise. It sounds to me like a high-pressure water spray or irrigation system… The hissing of summer lawns?
Now let's take it down a notch. Which gives us pink noise, of all things. Here things go truly mathematical ("a frequency spectrum such that the power spectral density is proportional to the reciprocal of the frequency" and more of stuff like that). But again, hearing tells the tale, so here's some pink noise for your listening pleasure. More subdued, right? Something like a medium sized waterfall… over there.
Given white and pink, we must — and do — have red noise, or, as it's also called Brown noise. This second name comes not from the colour but from Robert Brown, who discovered the eponymous Brownian motion. This, according to Wikipedia, is the signal produced by Brownian motion — random walk noise. Again, take a listen. The waterfall is much bigger and further away. Heavy rain on the roof, perhaps.
Should you want to have these masking noises for your own, to act as a slightly more grown-up version of sticking your fingers in your ears and singing la-la-la, you can of course download the MP3 files in this post and loop them in iTunes or whatever your player is. Or, you can go to SimplyNoise.com and have them played for you online. Choose your colour and your volume and 
Finally, here's a noise that you may not hear. It's known as the "mosquito tone" and it's used by children as their cell phone ring because adults can't hear it. PlasticMind Journal has a nifty tone test for you, increasing the frequency of tones until the 18KHz mosquito tone is heard — or not, as the case may be. This waterfall is very very tiny, and very very… very far off.
Don't rely on this mosquito tone test as a measure of your hearing. Some of the frequencies will be too high for ordinary computer speakers to produce.