How often do you clean your coils? Or change them? The gunk from dirty coils dissolves in your clean juice thus discoloring it. Also, if you vape at high wattages, you're more likely to burn the additives like sweeteners and colorings. Kanger recommends 5.5-7.5 watts.
Generally speaking, the dark and sweet juices will cause more crud buildup on the coils than the clean and clear ones; if that's what you vape, you'll have to clean your coils daily.