Just another voice to add to the pile of, "Yep, same here" and "it's probably the heat."
But, here's a new twist -- does anyone get a slight flavor change along with the color change? I've noticed with one of two of my juices that when the color gets darker, the flavor and throat hit also shift a just a nudge toward the harsh. Not to the point where it's something I'd complain about, just a sort of "on the edge of my mind" thing where it registers and then I can dismiss it.