I have multiple cartos, each marked with their own flavor. After I've re-filled them a few times, I un-mark them(pull off my little colored rubber band) and run them all
through the boiling cleaning method. After drying for a day in my dehydrator, I re mark and refill with flavors. My point is, once they're cleaned, I don't have any way of knowing what flavor used to be in them. But so far I haven't had any problems with any lingering colors or flavors. I use menthol, fruit flavors, caramel, vanilla, etc., and haven't had any problems with "cross flavoring" with this cleaning method so far.