I'm Sensing Some Confusion
So let's say you really want to taste purple or find a way to associate colors with numbers. How the heck does one get synesthesia? First, let's delve into the nitty-gritty of how scientists think (smell? hear?) how synesthesia works.
We don't know a ton about synesthesia, although it has become a popular field for study. Researchers do think the condition tends to be somewhat inherited or genetic, as about 40 percent of synesthetes report a first degree relative with synesthesia [source: Brang and Ramachandran]. Many synesthetes recall synesthesia as long as they can remember, and it can be discerned at an early age.
If you don't have synesthesia, you might be trying hard to make associations with colors or numbers related to memories. (You might decide the letter G is blue because you can recall a teal "G" on a poster in your childhood bedroom.) But synesthesia has been shown to be a sensory phenomenon, unrelated to memory. For example, if you are given a matrix mostly consisting of the number five all over the paper -- scattered with a few twos, which form the shape of a triangle -- you would have a hard time discerning the triangle. You'd have to look closely to search for the twos, and then discern the shape from there. A grapheme-color synesthete can see the triangle almost instantly. This has led researchers to conclude that synesthesia is quite real, and that it happens early in the perception process; your mind, in other words, doesn't spend time searching for a memory association [source: Ramachandran and Hubbard].
The more global theory of how it works is pretty straightforward: Researchers think that synesthesia is a kind of cross-wiring in the brain. In grapheme-color synesthetes, seeing a number simply stimulates your grapheme region and the area of your visual cortex that responds to color stimuli [source: Brang and Ramachandran]. One theory is that there are increased neural connections in the brain of synesthetes that could've been the result of less "neural pruning" while in utero [source: Brang and Ramachandran]. Even cooler is that there might be actual anatomical differences in the brains of synesthetes, like increased white and gray matter in the brain [source: Brang and Ramachandran].
Although one study did find that some exposure to color-letters built up their association, the effect didn't stick around [source: Colizoli]. So far, it doesn't appear one can "get" synesthesia.