The thing is, it was never about the signs.
For example, a quick perusal of this Civil Rights Chronology will show you that after the US Supreme Court ruled segregated public schools illegal, it was 3 full years before Little Rock, AR integrated theirs (and then the black schoolkids required armed protection from the US Army to get into the building). Then it was another 23 years later that the courts ruled it was legitimate to effect this by letting kids attend schools outside of their segregated neighborhoods. Housing discrimination, which enforced this neighborhood segregation, was still nominally legal until the Civil Rights act of 1968. However, integrated neighborhoods were still discouraged, both by real-estate agents and by irate neighbors. In Tulsa I lived in an integrated neighborhood in the mid 1970's, but it was the only one.
Signage is a similar issue. Originally the signs wouldn't have been much of an issue. Black people were just supposed to know their place. The signs came up after Plessy (1896), when courts basically ruled that facilities could legally be separated, as long as "equal" facilities were available. So the signs were the legal cover for refusing a facility to a black person.
After Brown overturned that in 1954, this bit of legal humbuggery no longer was valid. Many people who didn't want to serve blacks at private establishments that qualified as a "public accommodation" (eg: stores and restaurants), instead put up signs that read "We reserve the right to refuse service to anyone for any reason". The 1968 Civil Rights act made it illegal for such places to refuse service based on race, but the signs were still a useful marker (particularly in rural areas) for where blacks are not welcome.
But even without the sign, racists will find a way. Many people would for example just ignore a black "customer" in their establishment, until the person gets the idea and leaves. I had this happen to me at a national chain restaurant in rural Indiana in 1983 while traveling with some black friends.