Several suggestions on this forum are asking for some target highlighting functionality: To highlight allergens, identify non-Vegan foods, so on...
I would like to propose a generic solution to all of these: highlight lists. Users create/modify downloadable lists of ingredients to be highlighted. For example, a user could create a highlight list for "Gluten" which contains ingredients: "Wheat, Barley, Rye, Oats, ...". Then, another user can download the list and select to highlight all "Gluten" ingredients as green. Users individually select which highlight lists they want to show for their own screen only.
Users can then select to highlight whatever they wish. They scan the item and immediately see colors representing whether or not the ingredients have any concerning ingredients. Vegans may choose to highlight all non-vegan ingredients as red. Those with allergies, such as myself, could select a different color for each allergen to know which allergens scanned items contain.
In the future, other users can modify the highlight list for "Gluten" to add other ingredients (perhaps other languages) to highlight for. Some lists will be massive, even a minimal list of "Corn" allergens is almost 200 items long for english alone (http://www.cornallergens.com/list/corn-allergen-list.php).