Sarah Pocket came to the gate, and positively reeled back when she saw me so changed; her walnut-shell countenance likewise turned from brown to green and yellow.
– Charles Dickens
Great Expectations, Chapter 19. The metaphor of a “walnut-shell” to represent Sarah Pocket’s face is continuously used by Pip in the novel. The metaphorical language continues here with a description of how Sarah’s face color turns from brown to green to yellow, which it literally cannot do. This is meant to show how shocked her facial expression is.