Phosphorus deficiency?

is it cold at night? even upper 60s F can cause fall colors or augment a plant predisposed to showing red or purple leaves etc... Think this is genetics and or environmental... i'd becareful of blindly boosting p/k. you can quickly lock out other nutes. it's worth doing the math on what you feed to determine ppm of p and k in total - even for a soil grow. you don't need p much higher than 50ppm and K shouldn't be too much higher than 200ppm. now, those are soilless/hydro values... in soil it depends on numerous factors and potentially want to add less than that since plenty could exist in the soil still. either way.. if you add p/k boosting nutes and see issues start afterward, good chance it is locking things out. make a little note to use a bit less on next plant(s).