Ok, so I read the post all the way and re-looked at the picture,,,,,and,,,,,It is NOT P def!!! If it is any type of nutrient related cause.....It would be K. AND it's NOT a problem!
The purple being expressed in the plant can come from several things.
Night temps (May be a minor factor and only in relation to the K level , unless the 65F is hitting lower then you think)
Rise in K level/higher K level (This appears to be the ticket here in relation to the night temp swing and the strain) You can add some K at flower to express the "hidden" purple in some strains. Humbolt's Snow Storm Ultra does this very well. Plants that are being supplemented with high K silica's also tend to show this! I might venture that your using DynaGro and if your using their Pro Tekt. That would be the contributing supplement! (Interesting that your mixing the grow and bloom. I would like to see pics of the whole plant if you could-please).
The strain can express a purple on it's own. Some pheno's more then others! (I run a very Sativa Pineapple Express that with some prudent use of the Snow storm ultra, I get VERY purple plants at harvest.)
You don't say if your in flower (I suspect) or not. At any rate. What you've got going is OK. DON'T WASH! Keep going.