E-liquid Changing Colour?
If you've left your device for a long period of time, you'll quite often come back to it and notice that your e-liquid has gone from its original clear state to a darker, more brown colour. Or if you've left your device out in the sun, you'll notice this discolouration. This is a normal process for E-liquid, called "Steeping"
The Steeping Process
Steeping is where the flavours and bases of the e-liquid oxidise and become thicker and more potent. Quite often E-liquid starts as a light, clear colour and will shift over to orange, brown then to black. This process enhances the flavours and makes the e-liquid ever so slightly thicker.
Heat and Sun
During the summer months, the heat and direct sunlight will cause this process to happen much faster than standard. You'll quite often notice that you'll put your e-liquid into your device clear and an hour later it'll have darkened. This is a normal process as the heat and sunlight are causing the e-liquid to steep much faster.
When not to use it?
It's only once an e-liquid has turned black and become incredibly thick that an e-liquid is no longer usable. Normally this is after around 3-4 years. This is because the components that make up an e-liquid have become too thick for the coil to handle and will not soak into the cotton correctly or evenly.