My babies need help

Hello, i'm new here but must say that this forum kicks ass. I really hope someone can help me. Here is the deal.
I have outdoor two strains sweet tooth#3 and Grapefruit. They have been flowering for about 8 weeks now. Some are further along than others. My problem is that i noticed that on all of their leaves there are some purpuleish spots mainly on the tips of the leaves and some leaves are just straigh up yellow.

Also the stams of the leaves are reddish purple too. I'm very much on top of the feeding and keeping their ph balance at about 6-7. I have changed their nutrition when they started to flower. Before they were recieving 16 16 16 but half way through flowering i cut the nitrogen, could that have anything to do with it? Please help :P Thank you