I have been growing Thai basils by rooting the ones I bought from Asian mrket.

The color of stems can vary depending on the amount of sunshine; The more son, the more purple the stems will be.

The size of leave can also depend on growing conditions;
less sun, lots of water and nutrients can make bigger and more tender but les flavorfull leaves.

also there are actually 3 verieties of tulsi one is a red another is green and the third is a dwarf if you want it for medicinal make sure you have the red also the sun will help it turn color as well. ifn it dosnt change from green to purple or atleast hints of purple then you have the green veriety and that has less medicinal qualities to it i have both green and red

sorry i thought this was a tulsi question lol my bad and to anser you question yes sun does help it have hints of purple and also if id dosnt have an anise cent or flavor like liqurice then its not thai but thai basil is sometimes called holy basil. and it to has medicinal properties .

My Thai basil is currently grown indoors. The stems are green and they do taste like anise licorice. I visited a nursery today and realized that the basil that I actually want is cinnamon basil. They look and smell like the basil I normally purchase at the Asian store for Pho Noodles. So I purchased some seeds online and plan to flower a few to save the seeds for next year. This is such a fun hobby! Good luck planting your basil plants.