Cannabis plants are purple already, when exposed to cooler temperatures some of the chlorophyll dies, uncovering the blue/violet colors beneath. Strains like Blueberry and Mendo Purps are colored naturally trough genetics. You can cause deficiencies in late bloom that will give you good violet, sometimes maroon colors, this is done because the grower is focusing on maximum production and potency, not necessarily for the color. Sometimes when a pot plant has had a 'dry spell', the next time they get watered they will suck up more material than necessary out of a survival technique... maybe this could be used to suck up a food coloring or the baking powder? ...generally plants only intake nutrients and chemicals that they need/use, unless there is nutrient lockout/pH imbalance or irrigation problems.