If I could have any medical disorder...

... It would be Synaesthesia. Or preferably music->color synaethesia.

For anyone who doesn't know, a synaesthetic person has a disorder that usually links and blends two or more senses together causing them to attribute certain characteristics to their daily life that are not there. For instance, in ordinal linguistic personifcation (one form of synaesthesia), sequences like numbers, letters, days of the week, etc. are given personality traits, as in the following example:
"T’s are generally crabbed, ungenerous creatures. U is a soulless sort of thing. 4 is honest, but… 3 I cannot trust… 9 is dark, a gentleman, tall and graceful, but politic under his suavity" - an 1893 study of a man with OLP

In music->color synaesthesia, tones, timbres, and pitches are experienced as colors, or patterns of colors. I would liken this to constantly tripping on mushrooms (or whatever psychadelic of your choice). In fact, sometimes after doing drugs, people develop this so-called disorder (seriously though, this would be sweet).

Thank you to wikipedia for not only giving me this information, but also for making it possible for any idiot (i.e. me) to seem like he or she knows what he or she is talking about