I promise that I'm not trying to troll or anything. Were signs made by the businesses they hung in front of? Or was it something that was organised on a higher level? How good of a business was it? Did it impact the national economy during the time?
If you're talking about the signs saying "whites only" or "colored" over a drinking fountain, you're only talking about a couple hundred signs across the entire country. In that precomputer era, businesses needed handpainted signs for lots of purposes, and even a small city had a dozen sign painters listed in the phone book.