It refers to triage which in its original form for EMS divided patients up into those who were badly injured but not likely to live even if care was rendered, those who were badly injured but would be likely to live if care was rendered, and those who were injured and likely would live even if care was not rendered. Various agencies have assigned colors to those situations and expanded it to four and by my memory they are black for those who are dead, red for those who probably won't live, yellow for those who may live if treated soon, and green for the "walking wounded". They also maybe red for those needing immediate treatment and likely to live, yellow for the "walking wounded", and green for those having the appearance of being uninjured, but need evaluation to make sure. Some folks working in EMS would better be able to tell you the real answer to this, but as far as I know there is not a nationwide standard for triage and assigning colors.
