IIRC, there have been actual psychological experiments that showed this, too. As I recall, they dyed some sort of food, french fries and steak, I think, “weird” colors. The control group ate the meal under special light that prevented them from seeing what color the food was (to rule out the possibility of the dye affecting the taste or smell), and they said it tasted fine. The group who was given the meal under normal light found it completely unappetizing. And if I remember correctly some people in the control group actually felt nauseated when they learned the food they’d eaten was the “wrong” color.
I’m guessing the reason is because we’ve all been taught that funny colored food is probably spoiled. Maybe things like green ketchup works on kids because they haven’t gotten that conditioning yet.