why maud Martha keep silent in chapter 18, 25 and 29

Chapter 18: We're The Only Colored People Here
In this chapter, Maud Martha keeps silent because of a few reasons. First, she feels self-conscious, as she and Paul are the only African American theatergoers in attendance. Second, the other theater patrons appear to come from more prosperous socioeconomic backgrounds than Maud Martha and her husband. The men and women are lavishly dressed and look as if they have never lived in rat- or roach-infested apartments. Third, Maud remains silent because the patrons do not appear interested in engaging in friendly conversation. In fact, Maud observes that the white patrons look intruded upon.
Chapter 25: The Self-Solace
In this chapter, Maud Martha remains silent because Miss Ingram doesn't greet her and never engages in conversation with her. One can argue that Maud could have begun a conversation with Miss Ingram herself. However, Miss Ingram's obvious arrogance and rudeness makes the idea of a conversation with her distasteful. During the course of her conversation with Sonia, Miss Ingram also uses a racial epithet, which shocks Maud.
For her part, Maud remains silent because she expects Sonia to confront Miss Ingram about her rudeness. When Sonia does not, Maud has no recourse but to keep silent (since Miss Ingram's words were directed at Sonia). Because Sonia is reluctant to engage in a potentially contentious conversation with Miss Ingram, the insult is left unaddressed.
Chapter 29: Millinery
In this chapter, Maud Martha does not respond to the saleswoman's last question. During the course of their interaction, the saleswoman consistently displays her disdain for Maud Martha. When Maud Martha asks for a price reduction for one of the hats, the saleswoman tries to humiliate Maud for even asking. Meanwhile, the saleswoman suggests that a "lady of taste" would recognize the hat's value and therefore not inquire about a price reduction.
In the end, Maud does not argue with the saleswoman. She turns to leave. For her part, the saleswoman consults with the owner and then tells Maud that she can have the discount. Immediately, Maud announces that she no longer wants the hat. Maud keeps silent about the saleswoman's behavior because she understands that words will effect few changes. Instead, Maud chooses to show her displeasure and her hurt through her actions: she walks out of the store without purchasing anything. As a result, the milliner has just lost a sale. In turn, this may prompt the milliner to rethink how she treats customers like Maud in the future.