Can be caused by strains genetics, temps and nute problems.
There's loads of 'purple' strains. (I grow Potent Purple, myself)

Have heard of others putting their late-stage flowering 'purple' plants in front of the a/c to deepen the colors. Since wife and I are the only consumer of the end product, and I've heard it doesn't increase potency, I haven't tried. (but I will try it eventually lol)
Rusty Trichome Reviewed by Rusty Trichome on . PIC of The Dreaded Purple Vein Sativa! hahahaha here it is guys ... i aint complaining i love the colour through it, it loooks awesome and reminds me why i love plants so much ! peace guys Rating: 5