What is the importance of the theme "signs and symbols" in Chapters 17-20?

The theme of signs and symbols works itself into this section of the novel as Patti works with her nail polish. She’s conducting an inventory and looking at the colors she has in stock when Dell’s car, then the police car pulls up at her shop. She has always thought that red was a lucky color and has long wished it were the only color she carried. However, she’s holding a bottle of blue polish in her hand when the cars pull up. Even though she doesn’t yet know what news the people in the cars bring, she automatically decides that it is because she was holding the blue polish, and not the lucky, red polish that she will now have to deal with some trouble. Pattie’s superstitions are so strong that she decides to quit carrying the blue polish because she has decided it is unlucky even before she fully knows what is happening.