Synesthesia is a neurologically based phenomenon in which stimulation of one sense leads to automatic, involuntary experiences in a second sense. All very scientific, but what I really want to talk about is how people with this condition (which occurs in one of every 200 individuals) can, when they hear music, see certain colors associated with the sound. It's often described as something like fireworks, with voices, traffic, music and all sorts of sonic phenomena triggering the experience of color and simple shapes that arise and then fade when the sound stimulus ends. And different sounds elicit different colors, changing hue and brightness with variations in pitch and volume. Individuals with this condition often disagree about which sounds correspond to what color, but many agree on certain things, like how louder tones are brighter than dull, soft tones, whereas higher tones are smaller and lighter than low ones, and low tones are both larger and darker than high ones. One synesthesiac described the sound of an acoustic guitar as shades of yellow, while an elcetric guitar was bright red.
There are actually composers with this condition who have incorporated color into their musical compositions. Russian Composer, Alexander Scriabin was pioneering the multimedia performance as early as the nineteenth century and used his perception of music as color in the composition process. Rimsky-Korsakov, who was a contemporary of Scriabin, was a fellow composer with synesthesia and the two often disagreed about which colors were created by which notes (both maintained that the key of D major was golden-brown; but Scriabin linked E-flat major with red-purple, while Rimsky-Korsakov favored blue). Even modern composers have utilized light shows in their performances, matching the music to specific colors.
Being a visually-oriented person, I find myself wishing that I could see music too. Even as one not having the experience of the dual stimulus provided by synesthesia, I find myself thinking about which colors match which sound. I think it would come down to pairing the feelings evoked by music and those by certain colors. Irritating sounds might be orange, fast tempos red, slow, sonorous bass, blue. And I wonder how much influence the power of association might have in this situation...
Someone once told me that the sound of flip-flops was definitely yellow, but I would have to say they're a rather annoying shade of pink.