I'm crazy allergic to red #40 (I have to carry around epi-pens), and for the most part, WDW's safe. I've never had trouble.
Everything you get in table-service and quick-service restaurants is basically safe, although watch out for anything obviously red, of course (red velvet cake, dessert with red sauce, etc). Otherwise, there's nothing to worry about there. I like buffets, because it's easy to see what color all the food is, and there's usually a chef behind the buffet who can answer any questions.
Pre-packaged stuff is a different issue, though. If it comes in a package, read the ingredients! I'm pretty good at being able to tell if it has red in it before I even read the package, but watch out for the Mickey head-shaped ice cream sandwiches from the ice cream carts - those have red, but you wouldn't guess it.
I've eaten practically everything at WDW, so if you have any questions, just ask!
