As far as anyone knows, synaesthesia is a fundamental property of how some people's brains are wired, where the interconnections between two areas of sensory cortex are abnormally strong. Thus, when one area is activated (by perceiving a letter, for example), the activation spreads to the other area (for perceiving a colour). It's something that would develop very early on (probably in utero, and surely by soon after birth).
In adults, you might theoretically be able to induce permanent synaesthesia by repeated Tetanic stimulation of the two areas of cortex simultaneously. This may induce a strengthening of the connections between the two areas via Hebbian learning.
On the other hand, this might destroy your ability to perceive both letters and colour, or cause other brain damage. There would be no guarantee that the results would be of any use to you at all (remember that synaesthesics grow up experiencing the world this way). And that's assuming you could even identify the appropriate areas. Needless to say, no surgeon would do this.
