Christopher hates yellow because of custard, bananas, double yellow lines, yellow fever, yellow flowers (which give him hay fever), and sweet corn (because humans don’t actually digest it). He hates brown because of dirt, gravy, poo, wood (because wooden machines used to rot), and Melissa Brown (a classmate who tore one of his paintings in two).
Christopher doesn’t hate yellow and brown for the appearance of the colors themselves. Instead, he seems to hate anything yellow or brown because he hates certain yellow and brown things, which have swayed his opinion of everything in these colors.
Mrs. Forbes, one of the teachers, told Christopher that it was silly to hate yellow and brown. Siobhan, on the other hand, said that everyone has favorite colors. Christopher thinks they were both a little right, because it’s silly to hate colors, but it’s good to have something like colors to base your decisions on. He likens it to people choosing what they want from an unfamiliar restaurant menu simply based on the foods they generally like and don’t like.
Christopher exhibits some perspective on his own rules, since he admits that his hatred of yellow and brown is somewhat illogical. At the same time, he proves that there are common practices that are somewhat illogical, and he simply approaches his preferences from a color perspective, rather than from one of taste or name.