Here is a qoute from Sam's Blog:

 One thing that I still don’t under stand in the story is why he uses colors to determine if a day will god or not.  I believe if he wants to be more successful in life he should treat every day the same, making a day that might not have been great if he determined it by looking at cars, into a great day that he determins is a great day by giving it a chance.

     I believe that Christopher uses colors to decide wether or not it is a good has to do with how color is connected to emotion. Like in restaurants, when they have bright colors like red or yellow, this means that want costumers in and out. Color, for Christopher, triggers emotion that he can not understand. I all so think that when his mom went away, she was wearing something  that was yellow, thats why he has problems with that color. He must have some bad memory or bad experience that made him resent the colors yellow and brown. He states some reasons in the book, but they do not really go in depth into why he hates these colors. 
     Christopher uses the ideas of car color to judge a day because he struggles to find a pattern in his confusing world. He also counts and squares numbers to help him become calm and relaxed. He needs these patterns in order for him to stay comfortable on his life. As we can see in Mrak Haddon's book, Christopher has problems with being comfortable. He is constantly stressing about people touching him, or how many people are in one area or things that involve people in general.