I assume there was a point in time at which these signs were not "needed" as people of color already KNEW they were considered second class and didnt need to be told they wernt going to be served at a restaurant. Was there any particular catalyst for the appearance of these signs?
Bonus question: Which laws supported their legality?
While an interesting question, I would suggest also putting this in the Black History AMA going on right now!
The Plessy versus Ferguson Supreme Court decision in 1896 established the Separate but Equal policy in the United States and codified racial segregation. The signs you speak of, began appearing around 1890 in some Southern states and after the Plessy versus Ferguson decision, they became very common. By the time Woodrow Wilson was elected in 1912, they were everywhere in the former Confederate states. Source: http://www.lawnix.com/cases/plessy-ferguson.html