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.