The purple color denotes a lack of Potassium in the plant tissue, which can cause a drop in sugar production in the plant, possibly decreasing potency as well as flavor.
Most "stoners" think that purple means the pot is packing extra "punch". In reality (not pot induced fantasy), the purple color of the plant most often means either that:
1. It is of inferior quality and/or
2. that it does NOT increase the potency/flavor of the plant
Other causes for the coloring of the plants in temperature fluctuation, mostly when temperatures drop below about 55ºF.
So either way, purple does not mean the plant is better than the normal green plant, but that it is probably weaker than the another plant that is very green.
Stick with green kiddies...
"Actually that bud sucks, try this shit" -intellegent person
"oh shit, thats way better!" -idiot (gaining intellegence slowly)