The pics show up.
There are good purple phenotypes and ones that are not as potent as their green counterparts.
I've, like BrownTrout, also noticed their terpene profiles to be slightly different.
Perhaps the terpene differences can account for variation in potency observed by some people.
Or maybe it's like TomHill says (I think) that the purple plants don't always live up to their full potential due to lack of chlorophyll.
You just might be onto something there! Definite difference in terps between the different colored phenotypes in the lines I've seen. The green tend to be more bland tasting but have more "punch" or body to them, and the purple tend to be more flavorful with some subtle nuances to the high. Similar to what many people consider the difference between "sativa" and "indica".
I wonder if there was a green pheno to be found in say - Peyote Purple for example, how it would compare to it's sister?