Never seen this in any grows before, in general. Fans are starting to turn this super dark shade of purple, almost black on this durban (dutch x landrace), none of which is happening to its sister durban (with different pheno) or my other plant.
I can take more pics if interested but this one shows it in progress and fully “converted” on the big fan up top.
That one on the right kinda looks just like a dark green though, from the picture. The OP’s actually looks purple. I’m just saying I’d probably misdiagnose that as maybe too much N, if I had to, with only the picture.
I’ve found an addition of sulphur and mag to a feed at the right time will colour up better than any cold snap will. What you feeding at the moment? Unfortunately without you letting us know about environment and feed you will only get speculative answers mate