My wife's sweeter juices to the same thing, when she uses the same coil/wick without cleaning and dry-burning, or changing the head out. What I think is happening is the charred juice that gunks up the coil ends up leeching back into the juice that's in the tank, thus making it darker. Stay on top of your coils, clean and dryburn, or replace them before they get gunked up and darken up your liquid.
How often you have to do this depends on both your juice and your vaping habits. My wife tends to have this problem more them me, because she chain vapes more than I do, plus she uses much sweeter juices.
One thing about clearos is you have to properly maintain them to keep every thing, including flavor, clean.