Katrina is a minority character in the story. Yes, she is a minor character too, but she is an ethnic minority who cleans Miss Helen's house.
I would say that Katrina experiences two main forms of oppression that are illustrated in the story. The first form of oppression would be racial oppression. As a "colored" woman, she is subjected to the belief that she isn't as intelligent as white people. Even when Elsa is trying to defend Katrina's intelligence by telling Helen that Katrina can talk about more than cleaning and babies, it's still said in an oppressive manner.
Your colored folk read newspapers too.
Elsa might have good intentions, but the phrasing is clearly grouping Katrina apart from Elsa and Miss Helen.
The other form of oppression that Katrina experiences is marital oppression or domestic oppression. Her husband, Koo, is an abusive husband to her. Koo is a drunk that both physicall and verbally abuses Katrina.