That redness can either be a phosphorus deficiency or a natural occurence in some strains. What strain are u growing? What kind of nutes and how often?
Im not sure on the strain, but there are the odd purple leaf or purple tipped leaf. Im using AN 3 part with big bud, overdrive, pirhana, growzyme, annnnnnnd B vitamins. I folar feed every other day at 1/10 strength.
Funny, it looks almost like Molybdenum deficiency, which has red tips of older growth moving in towards the middle of the leaf, but if you are foliar feeding I doubt thats it....there is molybdenum in your foliar feed right?