ROY G BIV (a mnemonic for the colors of the spectrum -- red, orange, yellow, blue, green, indigo and violet) is a US$0.99 app that lets you point your iDevice anywhere, and the app averages the colors and synthesizes some notes based on the hues.
The keyboard covers one octave, from C to C. The colors determine the attack, decay, the oscillation and the waveform. The app also inlaces 4 sample percussive sounds, a snare drum, hi-hat, a kick drum and a barking dog(!).
ROY G BIV is simple enough to use. Upon launch, you'll see the keyboard layout. Point your iPhone or iPad at anything and you'll see the color cast. Then start using the keyboard to mix up the sound. I used my swimming pool, some books in my library, and my breakfast as color generators, and as you'd expect the sounds were always different.
ROY G BIV for iOS
What's the utility of this app? It's just a novelty really, maybe aimed more at kids who might get creative with it. I'd like to see some automation added, so the app could compose on the fly without any keyboard intervention, but hey, that's just me. This is a cute app, but I don't think it's sticky. Most people will try it a couple of times and move on.
ROY G BIV is a universal app, and requires iOS 4.3 or later, so it's a pretty low bar in terms of hardware requirements.