I Remember When I was like 14 My friends and I Were getting Probably midgrade bud from this kid but his weed was always really pink and purple.I do remember it having crystals...But the taste and Potency is nothing compared to the strains I smoke now a days.I've also seen Purple weed that was complete Shwagg.
like our friend said Purple buds has nothing to do with Potency.
And yes Some Strains Will go purple due to Temperature,(won't sativas do this in Northern climates?)
but many will go purple because it is a genetic trait,i.e.,Purple Kush,God Bud,Grape Ape,Purple Urkel etc...