Is there a particular brand or type you are questioning? Or did you just mean in general?
Basically any food flavor extract or concentrate comprised of Natural and/or Artificial flavors, Propylene Glycol, Vegtable glycerin, or food grade alcohol as the flavor carrier, and water will do ... just read the ingredients for the extract in question, and stay away from any with xantham or acacia gums in them.
I am ok with using extracts with color added, but I know there are many vapers who are not, so that one will be a personal choice.