What does "colored nurses" mean? [closed]

Colored was, two generations ago, an acceptable synonym in US English for what we now call Afro-American. In fact, the most prominent civil rights organization of the 20th century still retains its old name, the National Association for the Advancement of Colored People. But otherwise the term is no longer used.

So Atticus' remark means that Calpurnia never indulged the children she took care of, as it was believed that most African-American nurses—that is, she never allowed them to do or have things they should not do or have.

In other Englishes "colored" may have the somewhat different sense of "mixed-race".