Nirvana seems to have a lot of genetically purple strains... I've grown a few and had great success. Shame I dont have pics of my original grow setup :\
Chrystal: NL x White Widow. Two pheno's that I observed. One green and slightly spindly, the other deep purple and a bit hardier. Both taste fruity with fuel/kerosene undertones like the widow, but grow and flower like northern lights.
ICE: Shiva x Skunk x NL x Haze. Two pheno's again. Both pretty much identical. One is purple the other is not. The green one is a bit more on the indica side and bushy, the purple variety is a bit taller and more sativa. Never kept the purple one cuz it sold so fast but the flavor of the green was pine/diesel.
Bubble-licious: Afghan x Bubblegum. Once again, two phenotypes that I identified, and both polar opposites. One stays stout, green, and nugs up very thick but smells like choclate and tastes like bubblegum. The other gets pinkish/lavender hues, grows like a bubblegum plant, smells like bubblegum, and tastes like choclate.
Northern Lights: Need I say more? Sh*t turns purple pretty easy with temp adjustments or even on its own at times, depending what particular geno/pheno/strain, and/or who bred it.