Looks genetic to me, and looks more red/orange than purple. I have some Purple Kush that is green until you flower it and then all the new leaves and calyxes grow purple. The PK is also very succeptible to nute burn too. But yeah it looks to me like the color change is natural. They yellowing is also probably normal as you are on the down slope of flowering time so to speak.

BTW, here is a good thread for nutrient problems you might have. I bookmarked it!
The Complete guide to Sick Plants,pH, and Pest troubles! - International Cannagraphic Magazine Forums