Although OSHA (Occupational Safety and Health Administration) notes the color and lettering of exit signs must be distinct, there is no law or regulation in the United States that mandates a specific color.
Green and red are the predominant colors that are used for exit signs, but ultimately State and local authorities have actual jurisdiction over exit sign requirements such as size and color.
Advocates for red argue that red is the most visible color, typically used to attract people’s attention, while the green team reminds everyone that green means “go.” That said, if you live in Helena, Montana, you will notice that exit signs are in orange. Not sure how they picked that color, but we must assume (or at least hope) it is effective.