My Thai basil has purple stems but small green leaves, much smaller than the Genovese. I'll try to hunt down my camera this weekend and post a picture, since it sounds like that might be on the right track. I bought it as a small plant, so not sure when you start the seeds. It's doing well now, so I'm guessing it's a plant in the spring one. I found a link to a few different kinds. Mine looks the most like the Thai holy basil here:
http://www.evergreenseeds.com/thaibasil3.html