sry it took so long for me to find this one, hopefully its not to late. when your plants are ready to flower they require much more Phosphorous. when they are deprived they with turn dark green and sometimes a hugh of purple. luckily its not fatal to the plant it just stuns growth a bit. just get a fertilizer high it (P) Phosphorous, but note that it will not fix the leaves that already have been damaged just the new growth.

hope this helped...peace :rasta: :rasta: :thumbsup: