Synaesthesia is fun, and I think synaesthetic magic would be splendid.
I've often thought it could also be the starting point for an explanation of magic in a subset of the population. There is evidence that suggests all babies are synesthetic, and perhaps it's more accurate to think of some people as
keeping their synaesthesia while the rest of us lose it; a similar (related?) thing could happen with magic.
There's also a correlation between synaesthesia and art, and well, there's always been a link between art and magic, right?
Professor Ramachandran at UC San Diego makes big claims about synaesthesia.
[font=Arial, Helvetica][size=-1]“Synesthesia might tell us how the brain makes metaphors, which often take the form of cross-sensory associations – think “loud tie” or “sharp cheddar,” Ramachandran said. “Processes similar to synesthesia may underlie our general capacity for metaphor and be critical to creativity. [/size][/font]
[font=Arial, Helvetica][size=-1]“It is not an accident that the condition is eight times more common among artists than the general population,” he said. “A quirky color/number synesthesia is not on the evolutionary agenda – but the ability for metaphor, a flair for connection, is. In fact, it’s one of the hallmarks that makes us human.”[/size][/font]
http://www.sciencedaily.com/releases/2005/04/050411210623.htm
(Rama makes overly large claims sometimes. But most of the synaesthesia work was actually done by Ed Hubbard, who's cited in this article. Ed was one of Rama's students, and it was his dissertation work. He's a friend of mine, and I can vouch for him being a really rigorous experimentalist... uh, not that I have any particular rep here, so you can also ignore my vouching if y'like).