This is interesting and so common most of us probably don't even notice. From the website Famous Logos: Take a look around and you’ll see that most fast food logos contain one or more of the following colors– red, yellow, orange, or green; particularly the former two.
Such an obvious observation but do we ever give it a thought? Look at this link which is filled with photos of fast food restaurants and it's amazing how red, yellow, and orange they all are. So what is with this color copy catting?
"According to the color theory, these colors are known to subconsciously trigger hunger and/or induce excitement. These colors encourage guests to spend more and leave quickly– which is exactly what fast food restaurants want you to do." And people eat more in rooms with these colors as opposed to rooms colored with blue, black, purple or even white.
I can think of a couple of fast food places which have not succumbed to the red and yellow-White Castle, Culvers, and maybe Steak and Shake although Steak and Shake does add red as an accent color so they are probably much like the others.