Yes, I agree this is happening because of p deficiency, but I don't think that every strain that does this has purple genetics.
I once saw a chart explaining why most leaves turn yellow when dying, and on some plants they turn purple (and sometime that pretty red). I did a quick search and couldn't find the chart but it explains how certain deficiencies cause different coloring. So I don't think the plants turning purple have any purple genetics, just when they get certain deficiencies they will exhibit the purple color.
And again to clairify, the purpling from deficiencies will never turn the entire plant purple like some of the purple buds shown in the thread. Those are purple genetics, clearly.