Over time, (usually within a few weeks) the nicotine in your e-liquid reacts with the flavourings, giving the liquid a deeper yellow/gold hue. This is typical of all e-liquid, including the e-liquid found inside pods, and just gives you an indication of when it was made. This is why nicotine-free e-liquid is always clear (unless food colouring has been added).
There is no danger or cause for concern with a change in colour—in fact, the flavour tends to be better once it has changed colour because it has had time to steep.
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