You're well on your way to the finish line and your plants are showing you this, they can turn many colors during the ripening period and purple is one of them, if you have a look at my LA confidentials you'll see some beautiful purples from my girls, you're doing fine and likely your night time temps are dipping lower to 16 or 17 and that's another factor for leaf coloration, some growers intentionally lower the temps like myself to get that coloration....Good luck, they look fine and healthy..