No pic provided but not necessary for this. Purple in the plant is a pigment called anthocyanins. If you are looking for a human equivalent, it would be melanin that affects skin color and in a similar way, determined primarily through genetics, and then influenced by environment. It serves a function. The anthocyanins make the leaves darker and allow the plant to absorb more heat in colder weather. As it turns out cold roots is one of the triggers that will bring that purple out. The nutrient that's going to have the most interplay with anthocyanins is phosphorus so if uptake is off, the anthocyanins show up. And the most common is when phosphorus mobility is hampered by cold root conditions. But it's not the only one. The cation can be off, salt stress, etc can also trigger their expression. The only time you want to sit up and take notice is if and when that purple suddenly shifts to some of the leaves, and then you want to think is it cold, are you leaving it too wet or is there an unbalanced nutrient load or problem with cation.
