Sometimes different flavors are different colors (like, darker for example). With that said, there are also juice makers that use coloring in their
e-liquid I think? I'm not a fan of the practice, it just adds an extra ingredient to gunk coils and that I am not entirely sure are safe to vape (though I think if there were a HUGE problem, we'd be likely to have heard about it.
I DIY
e-liquid, and most of mine ends up either clear, or slightly yellowish. Age can also cause e-liquids to darken, but I don't know what would cause a blue tint other than food coloring.
Hope this helps, you can always ask juice companies beforehand if they use coloring in any of their particular juices.
Anna