Can't be the cold since they have seen some really really cold days and never had this color.They look almost like theyre turning purple?
Could be phosphorus, cold, or just genetics.
Is there a phosphorus supplement I can give?Could just be a different phenotype that has slightly different needs if theyre all from the same seeds and only a few are doing it. It might not like the colder water temp. Ive nearly always seen purple that wasnt supposed to be there blamed on phosphorus or cold and the two share a pretty close connection.
I thought this at first too but couldnt find any pictures of any NL autos with purple fan leaves. After another search I am seeing some mentions of it on forums.I honestly don't think its anything other than genetic expression. If the plants are trucking away and seem fine, it looks more like a blemish or lack of chlorophyll in those acute areas. Nothing to be to concerned with mate. Keep an eye on it, and if it gets worse I would call a pot doctor Joking aside, it isn't anything to freak about.