Did they turn purple out of the blue? this could be a (P) deficency, or these could be an indica trait, one clue could be if this shows up in other parts of the plant at all, like the leaf stems, it will start in the stalk most likely and move in to other parts, but indicas will sometimes already have these colors. What strain is this, do you know? Sativas wont normally do this naturally, Indicas can which is ok. Alot we work with nowdays is all hybrids which makes stuff like this harder to pinpoint. Have you been feeding them good (P)? Is ph ok? If all this is checked, than you can look at the strain and if this is a natural tendency. But if this is a sativa I would really check out the (P) issue.