Is it ok if vape juice turn dark?

In most cases, the blackening of e-liquid is completely normal and can be safely used. This is mainly caused by oxidation, caramelization or maturation processes.
The blackening of the e-liquid can generally be attributed to the following situations:
Normal phenomena and safe usage conditions
The bottle of e-liquid turns black (oxidizes): This is the most common situation. The e-liquid contains nicotine, which is highly reactive and will undergo a chemical reaction (oxidation) when exposed to oxygen in the air, causing the color to naturally darken. This process is harmless and does not affect the quality, safety, or taste of the e-liquid. The higher the nicotine content, the darker the color and the faster it darkens.
Turning black (caramelization) in e-cigarettes/vaporizers: To enhance the taste, most e-liquids contain sweeteners. When these sugary e-liquids are repeatedly heated by the vaporizer core, they undergo a caramelization reaction similar to sugar heating in a pan, forming a dark brown viscous residue, which is what we commonly refer to as carbon deposits. These carbon deposits dissolve back into the e-liquid, making it darker and darker. Although this process accelerates the formation of carbon deposits, it does not mean that the e-liquid itself is toxic.
The color of the oil "matures" and deepens: Some tobacco oils will naturally darken in color after being stored for a period of time. This process is similar to the aging of wine and is called "maturity". It can enhance the blending of certain flavors (especially those of cream, tobacco, and baked goods) to a better extent, and the taste may even be better than when it was newly opened.
2. Warning signs that require vigilance and should be discarded
Although a darker color is generally a sign of safety, if the e-liquid shows any of the following signs of "spoilage", it must be stopped from being used:
Signs of deterioration Detailed description
Appearance of suspended matter or sediment Visible, floating solid particles or cotton-like substances can be seen in the e-liquid.
Elevated turbidity of the e-liquid Originally clear e-liquid becomes cloudy and opaque.
Production of irritating odors Instead of the normal sweet smell, unpleasant odors such as rancidity, putridity or metallic taste appear.
Layering occurs There is a clear separation of oil layers in the e-liquid, indicating that its chemical structure has been damaged.
Significant change in viscosity The e-liquid becomes extremely thin or overly thick, which usually means significant changes in the composition.
Exceeding the shelf life All legally sold e-liquid bottles are marked with an expiration date. After expiration, it is recommended not to continue using them.
In summary, if only the color of the e-liquid becomes darker, but none of the above signs occur, then it is still safe. You just need to be aware that darker e-liquid often means faster carbon accumulation, and it may require more frequent replacement of the atomizer core.
If you want to learn more about how to properly store the e-liquid to slow down the color change, or how to clean the carbon deposits in the atomizer, I can offer you some relevant suggestions.