The Teal Pumpkin ProjectTM is a movement sponsored by the FARE (Food Allergy Research & Education) organization. It encourages people expecting trick or treaters to place a teal-colored pumpkin or Jack-o'-lantern in front of their house to indicate that non-food treats are available.
The goal is simply for kids with food allergies to have a happy Halloween -- a chance to go trick or treating and know that they will get something that won't make them sick. However, if you buy enough for everybody you might find that most kids are happy to get a mechanical pencil or a pair of vampire fangs in place of another Dum Dum pop, and you can also join the fight against cavities.
Teal, of course, is the color of food allergy awareness. While you are encouraged to get a real pumpkin and paint it teal, there is also an optional poster for your window available for download. You can also register your house on the website, so that kids with allergies know that you're ready for them.
While the teal pumpkin, like trick-or-treating itself, is primarily an American meme, FARE's site indicates that there are a few participants in Canada and Ireland as well.