Why do u want the plant to be purple? I mean its just a phenotype, has nothing to do with anything worth while. If u really want a purple plant grow one with the gene, Like purple urkle or something like that. If u over feed it with potassium it may turn a bit purple, I seem to have that issue all the time, for some reason my stems are always purple, and i get slight purple coloring in the buds. However, some strains under the same conditions just turned purple on the stems and nothing else. Temps never below 70