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
MasterKushy Reviewed by MasterKushy on . HELP! Purple! There are a few plants grown indoors from bag seed about 4-5 weeks into flower. One of them started turning purple. The rest of them are still green. The temperature has never dipped below 16^C at night. What's going on here? Photo 1 & 2 The plants have been growing in a Fox Farm soil that came loaded with nutes. If any fert/nutes are used (even diluted to 1/8) it gets really bad nute burn. For this reason they've only ever been fed a few times with poor results. In addition to Rating: 5