Yellow and its different shades are used in the novel to set up a contrast between the West Indies, Lucy's homeland, and the United States where the girl is working as an au pair for Lewis and Mariah. Thus the color symbolizes cultural difference and is mainly associated with the family where Lucy is working, Mariah and her favourite flower, daffodils.
The day after her arrival to the States, Lucy notices that sunlight is extremely different from her home country. It is a "pale-yellow sun" as contrasted with the "bright sun-yellow making everything curl at the edges" that she was used to (p. 5).
While her skin is "the color brown of a nut rubbed repeatedly with a soft cloth", Lucy describes the photos where Lewish and Mariah's family are standing together with their children as "six yellow-haired heads ... bunched as if they were a bouquet of flowers tied together by an unseen string" (p. 12). Yellow is particularly associated with Mariah to establish the divergent cultural, social and geographical milieu that the two women inhabit. The narrator frames her in "almost celestial light" with "yellow light from the sun" spotlighting her while she stands in her kitchen whose floor and walls are shades of pale yellow just like Mariah's hair and skin (p. 27). Mariah's description emphasizes her spirituality ("she looked blessed") but also how fragile she is because of her adherence to social expectations and conventions.
Mariah is particularly fond of daffodils. Once again the color is used to set up a contrast between Mariah and Lucy. While Mariah finds daffodils pleasant, Lucy is reminded by a poem she was forced to recite in front of her all school, an experience that gave her nightmares the following days. The reader can obviously think of Wordsworth as the author of the poem, which brings up interesting questions about colonial and gender domination. Lucy's dream where she is chased by daffodils and finally overwhelmed by them almost reads like a rape scene (p. 18). Mariah's passion for daffodils can be read as her acceptance to be defined according to a male point of view.
Page numbers refer to the Farrar Straus Giroux 2002 paperback edition.