A root system is similar to our blood/brain barrier - only good stuff gets filtered and allowed through (assuming a healthy plant) and thus trying to dye the plant doesn't work unless it's bound to something else the plant will allow through.
The stuff they use to make your stuff turn purple is exactly that. And you can tel it gets used when you put your bud through the water test - take a cured bud, break it up in your hands, and run water over it - stuff flows like ink. Anytime I see purps like that I ask for my money back and never speak to that person again unless they have GREEN weed, because they obviously can't tell genetic or aged purps from forced purps.