It's normal as far as I can tell. With the tank, you can see what's going on inside, so Lord knows what color your juice is turning when it's inside a carto. I've heard a couple of explanations, and these might not be the best, but one is that PG burns off at a faster rate than other parts of the liquid, so you've basically got a concentrate at the bottom, which would be darker. The other is that perhaps the heat that it's exposed to as you're vaping away changes the color. I like this explanation because heat speeds up chemical reactions. Your juice will generally turn darker colors as you store it long-term too.