I have grown a lot of green basil in my life. I've had less luck with purple basil until last summer, when I was able to grow several basil species, including green (genovese) and purple (amethyst I think).
I guessed it was a mutation from cross-pollination, but a biologist friend told me that it's more likely caused by a virus, like the tulip breaking virus.
Is this common? Is the cause known?