Have you noticed that some e liquids tend to chainge their colours (whether in clearomiser or when stored for a while)? Don't worry, you don't need to chuck it away. It's a natural chemical reaction and it does not affect its quality or taste.

This process is known as oxidation. Nicotine is a very reactive substance, and can cause e juice to change colour either in reaction with other ingredients in the liquid, or after being exposed to air, light or heat. Nicotine-containing juices tend to become darker over time, even when stored in cool, dark places, but some companies add small amounts of preservatives or stabilizers to prevent oxidation. Storing e-liquid in a cool, dark place can slow down the oxidation process, but it will never stop completely.

Another cause for the darkening is that the wick and wire become coated with caramelized e-liquid.  This dark caramelized e-liquid essentially "stains" the fresh e-liquid that was added to the tank. Also, the heat (that is being created to vaporize the liquid) has a similar affect on e-liquid as air does in changing the color.

Since this “caramelization” ultimately affects not only the color but also the taste of the e-liquid, it’s recommended that you clean both the tank and the atomizer before refilling. Of course, cleaning can only do so much, and you’re eventually going to have to replace the coil and wick of your atomizer, or use a brand new one, if it’s a disposable. It’s worth remembering that gunk buildup is closely tied to the sweetness and viscosity of the e-liquid; for example, a thick, sweet juice will caramelize much faster than a PG-based tobacco e-liquid.

Whether caused by oxidation or caramelization, a change in the color of your e-liquid is nothing to worry about. It’s actually a common occurrence, and if it hasn’t happen to you before, it’s probably because you’ve either been using nicotine-free juices or clear, PG-based unsweetened ones.