Hey Ken, was talking about this in another thread a few days ago. I found this video on youtube from Jorge Cervantes that explains the colors a bit. Pretty cool stuff.
He talk's about why it get's it's purple color at 2:45.
Here's an article someone posted in the other thread from High Times
Purple Cannabis
The origins of purple cannabis are as much of a mystery as the origins of the plant itself. However, purpling is as natural as the changing colors on the leaves of deciduous trees in autumn, which is attributed in part to the pigment anthocyanin. Anthocyanin expression is controlled by both genetic and environmental factors. Purpling is a simple dominant Mendelian trait, with quantitative expression.