It seems kinda pointless to me too, to color eliquid, but if you really want color, you might try the Lorann flavors that contain coloring -- it's more for food flavoring than eliquid (they also have many of their flavors without color, which is better for vaping). The fewer chemicals your coils have to deal with, the longer and better they'll last and perform. But I've used some of the Lorann flavors with colors, and they vape ok, if you don't use too much flavoring.
Just be aware that it's just one more thing you're inhaling into your lungs, when the whole point of vaping instead of smoking was to inhale fewer chemicals into the lungs, some of which may be quite toxic -- to inhale, if not in general.
Andria