In chapter 131, Christopher explains why he hates the colours yellow and brown. For example Christopher hates custard, sweetcorn and bananas (bananas also turn brown), because these things are all yellow. The reason why he hates brown is that they remind him of dirt, gray and poo.
He also tells that one of his teachers, Mrs Forbes, says that hating yellow and brown is stupid. But his mentor Siobhan doesn’t share the opinion of Mrs Forbes. Because Siobhan thinks that everyone has his favourite colours. Christopher thinks that both of them are right, because everybody has reasons why he hates some things. He compares this with a menu in a restaurant. He explains that you would never choose a food, which you never tasted before, because you don’t know if you will like it, so you have favourite foods and you choose these and that’s simple.