Bout a year ago I racked a lambic into secondary and added cherries. With the carboy filled I had a half gallon or so of extra lambic (unfruited) which I racked into a growler. It was a pale straw color. Lately, I noticed that it has darkened considerably, now its more of a copper/red color with a thin white film on top. Smells fine, mostly lactic sour at this point. Just wondering if anyone knows what’s going on with the color change?
Oxidation over time, and/or the acids produced are reacting with some compounds to form pigmented species. Its pretty normal. Also possible that the pellicle is making the stuff look darker and it will be lighter when you bottle.