you are right try briging down temp at nigh it will definitely help whit the purple it did the job on my "Smooth Smoke strain" you can check it in my diaries
Your conditions are good. My plant was ready in 62 days and has purple sativa phenotype just as yours. It has great sativa effect and all if my friends loved it. Good luck.
@RyukyuFarmer, my plants gave sativa buds, they are airy and not dense. For first time I regret that there would be not great harvest, but after I've cured and tasted these crystalic buds this strain become one of favorite. From one plant I took about 30gm.