Teal is the color of allergy awareness and putting a teal pumpkin out signals that you have allergen-free or non-candy treats available to trick or treaters who need them.
Halloween is very different than it was when I was a kid. Now kids Trunk or Treat or Trick or Treat in malls. We have to have nut-free, gluten-free candy to hand out, and Halloween parties aren't allowed at most schools. Dressing in our Halloween costume for our school party was the highlight of the fall for me...
Here's a list from Ranker of the worst things to get while trick or treating. Not the worst candy, the worst stuff! Some folks don't get the whole "treat" aspect of the holiday.