FYI, that's anthocyanin (with an 'a' in there) that's the purple pigment maker. Here's more info
than you probably wanted to know.
In fact, on the discussion of the original of autumn colors (coming from absence of chlorophyll) the statement "While this is indeed the case for the carotenoids and xanthophylls (orange and yellow pigments), anthocyanins are not present until the leaf begins breaking down the chlorophyll, during which time the plant begins to synthesize the anthocyanin, presumably for photoprotection during nitrogen translocation" would suggest that the previous quote about anthocyanins would mean that purple tea was harvested from plants about to go dormant for the winter.
The funny thing, though. . . I think it was on PuerShop where they mention one of the purple pu-erhs has a nickname of a "triple tea" -- because the leaves are green when cut, purple when dried/pressed, and amber when brewed.
Sounds like a lot of vendor-bunk flying around.
Edit: eh, after thinking about this more, some plants *do* obviously produce this purple pigment before autumn (blueberries, probably violets). So... perhaps particular sub-species do this so that after the kill-green step, all the purple shines through? Hrm, if I were still in chemistry, I think I know what I'd focus my studies on. . . .!
Of course, that's assuming all of this was a horrible butchering of "purple" into "pepper"....