Mosherd7answered grow question 4 years ago No worries. Looks very healthy, probably caused from low phosphorus. So much gets used in flowering.
But if its just showing this late in flowering, you are fine, lots of people flush the last 2 weeks anyways. It'll just drain your fan leaves to start.
Cold can also turn them purple but unless its below 5 degrees I never noticed much purple before. A few light frosts outside could do that too though.