If you were to take a look at the color wheel, you’d notice that green is positioned opposite red. There is some theory to support why this makes green an ideal color for walls inside an operating room.
Typically, a surgeon may spend anywhere from a few hours to many hours looking at the innards of the person they’re operating on. Peeling their eyes away from all that red and focusing, if for a few seconds, on an opposite and cooling color like green can help give their eyes the ability to effectively refocus on what they’re looking at when they turn their eyes back to what requires their attention, per University of California, Davis professor John S. Werner, who studies neurophysiological computations and mechanisms that mediate human vision (via Scienceline).
Also, as medical expert and U.K.-based junior doctor Dr. Ollie shared on his website, green is a good color to combat the afterimage phenomenon. Remember that time when you were staring at one color for a long time and took your eyes away from it to look at a white wall and ended up seeing traces of the color you were looking at before? “Surgeons may experience this same phenomenon if they’re concentrating on the brightly lit blood and tissue in the operating field and then go to look at their colleague’s white scrubs.”