That is definitely enough of a temperature swing/low enough temp to do it if it is in their genes. Purple colors are only really good for upping the bag appeal to novice smokers. Actually, (according to subcool) the purple phenos of a given strain are nearly always less potent than the "standard" color phenos.
As far as the product you're refering to, I can only assume it's Humboldt County's Pruple Maxx (aka Stacker). I actually got some as a gift, though I'm yet to try it out. The only thing is, the manufacturer is the only source I can find saying that it has actually brought out purple colors in a plant...and nute (or supplement) manufacturers are kind of notorious for exaggerating (read: making up) the awesome powers of their product.
If its in the genes, with the temp changes you have...especially how they will be as harvest time grows closer, if the colors don't come out naturally I wouldn't expect any product to make it happen.
Hope that helped. Good luck, man.
