The specifics are based both on historic precedent and the sign language of colors. Yellow, for falls, is commonly understood as a symbol for caution, whether by janitors mopping floors or by traffic engineers. Red, for allergies, is associated with a command to stop and check for danger. In the past, some hospitals have used blue to signify D.N.R., but it was ruled out on a national level, Ms. Tena-Nelson said, because the phrase “Code Blue,” is often used to call for resuscitation, the opposite of what D.N.R. patients want.