Sometimes, e-liquids turn dark while still in the bottle. This is due to oxidation between nicotine and oxygen. This chemical reaction causes e-liquid to darken, though keep in mind this will not affect the flavour or how it vapes. Exposure to light and heat can also gradually cause the nicotine in your e-liquid to change colour. VapeMeet recommends storing your e-liquid in a cool, dark place.
On the flip side, when e-liquid in your tank or pod when exposed to heat, the sweetener in e-liquids will caramelize and can change the colour and texture of the liquid. This caramelized gunk can also stain the existing e-liquid in your device. Over time, it will build up on your coil and seep into the tank, leaving residue behind. Without regular cleaning, this buildup will start to darken any new e-liquid added to your device as well. This is a good indicator that your coil could use a change, ignoring these signs can lead to a loss in flavour or performance.