For sure man, We are kinda stating the same point, I think. Correct me if Im wrong, you are stating that the red is from a nute issue or a ph issue. I am expanding and saying that its a sign of stress. Stress from a deficient, ph, pests, drought, and the excess metabolizing that liquid light causes.

Attachment 283435Attachment 283437Attachment 283436 last photo has no reding

For some reason a flush with azamax missed up a couple of plants. Other than lack of vigor in the leaves some blood red at the stem showed up.... in one of the pics another plant is still healthy and green. It didnt get stressed like the other two did. Not real sure why... I didnt mean active damage from pests, like one would see from mites. I meant that if one has nats the larvae feeding on the roots with stress the plant out. One way to see this is when that purple/red shows up. But its not limited to pest, just stress. I hope that helps!