I am a born again citizen of the developing world and I will prove to you that the color of my life is gray. (1.36)
Gray, as we learned in preschool, is what happens when we mix black and white together. It's also a common symbol for anything that we can't quite place neatly in one category or another. In the case of Little Bee, her identification with the color gray suggests that she feels numb and muted as a result of her experiences.
Gray is also a symbol of the hybrid identity she's developing. Because of her circumstances, the development of this mixed identity has been really uncomfortable. Had she spent her first two years with, say, Sarah and Andrew, instead of in the immigration detention center, the experience might be way more positive for her. Likewise, if she wasn't so acutely aware that she, an immigrant, is not welcome in England, she might not feel like such a monstrosity. But, as it is, she feels like a freak, somebody who belongs nowhere.
It's important to keep in mind that this is only how Little Bee sees things some of the time, just one aspect of her feelings about her life. And, there's some suggestion that she's beginning to regard her hybridity as a beautiful thing, something that doesn't result in gray, but in fresh, vibrant, living colors. For example, she gets extremely hopeful and excited whenever she sees people of different skin colors interacting. Remember how happy she is when she spots a bi-racial couple and their child in London? Or when she substitutes her black finger for Sarah's missing white one? Or, the final moment of the novel – children of different colors playing together? There are some bright colors on the horizon after all.