Symbolism, Imagery, Allegory
Christopher has a system for determining what his day will like, and it all depends on the color of the cars he passes on his way to school. This system is definitely representative of the order and structure that Christopher needs in his daily life in order to not be in constant freak-out mode.
But then, one day, his system gets disrupted:
And the next morning I looked out of the window in the dining room to count the cars in the street to see whether it was going to be a Quite Good Day or a Good Day or a Super Good Day or a Black Day, but it wasn't like being on the bus to school because you could look out of the window for as long as you wanted and see as many cars as you wanted and I looked out of the window for three hours and I saw 5 red cars in a row and 4 yellow cars in a row which meant it was both a Good Day and a Black Day so the system meant it was both a Good Day and a Black Day so the system didn't work any more. (229.43)
Christopher realizes that the rules of his world are no longer useful in this scary new world in which he finds himself. But more than that, this sudden disorder symbolizes the chaos that has taken over in his life where order and logic once reigned.