Over time (usually within a few weeks) the nicotine in your e-juice 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 the change in color—in fact the flavor tends to be better once it has changed color because it has had time to steep.
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