I don't know about phosphorus deficiency; but I do know that lots of plants have purple genetics floating around, which only surface once in a while(mendel's peas). sometimes you'll get one or two that may be purple stem to the dicot and no higher, or partial purple.you see it a lot more of this outdoors. like my avatar sgt.kahneohe...she's a kahuna from soma..now you wanna' see the most beautiful weed, go to soma's website. my kahuna cuttings turned mostly RED! ...and yellow and orange and purple.