Red and yellow are colors we associate with hunger. That's why McD's chose those colors. Just as green is a "peaceful" feeling so Hospitals paint thier walls green. Just as autistic clinics and PreK's paint thier walls orange because it evokes creativity
Red is also one of the hardest colors to read. Companies that want to make you hungry will probably use orange, which is associated with hunger. Places like doctors offices where they want you to feel calm will probably use pink. Color association is so interesting.