We get this question a lot.
Vape juice naturally changes color over time so, depending on its birthday, you may find that the exact same vape liquid may look different from one order to the next. This is because of the presence of nicotine. As nicotine oxidizes, it darkens gradually.
You can see this in action in the photo below:
The bottle on the left is freshly made, whereas the bottle on the right is three months old. The change in color is due to the natural oxidation process the nicotine is undergoing so there's nothing to worry about if you notice some color variation from one bottle to the next. This is a normal process and to be expected.