Your plants look very healthy, and that actually looks like pigmentation rather than a disease. It's really hard to say, not knowing too much about your strain and conditions though. Some plants do go purple in cool temperatures, especially the bud sites.
However I'd be concerned with monitoring it to see if any necrosis develops- if it goes from purple to brown, you've got problems! One thing to check first of all is that there is no reason to suspect root rot, such as leaf droop, drainage problems, or a foul smell under the pot.

I tried to zoom in on the pic and can't get a closer look, but check that the dark spots aren't softer than the rest of the leaf, and that the leaves they are on have a normal shape and rigidity.