Yeah, low temps will quickly turn your plant different colors -- dark purple / red etc. Will look like fall... for same reasons.
it is related to how it impacts nutrients, but you want to fix the environment as opposed to changing nute mix. 18C is borderline... some strains may still turn at this temp too. Once it starts going, it doesn't take many nights to speed up.
good idea bringing it in... will be a hassle of course... late in flower can just live with it if you want. Usually get soem pretty pictures t the least, lol.
The advertised bloom periods are almost always a fib. plus, about a week give or take just based on personal preferences. (about comment from diary) Definitely experiment with different points of harvest window. use trichome coloration as guide. Get a 30-60x jewler's loupe or currency checker etc... build up familiarity.