The correct answer is, the main purpose of telling how the colors change for Jonas is that Jonas wishes other people to see the world the way he sees it. That people can see that they can see in colors and that they can decide among different things.
In Chapter 13, Jonas has learedt to see the different colors and he wants to share this information with other people. Another important consideration is that having the freedom to decide can carry to many undesirable consequences. Anyway, Jonas considers that people should decide on their own.
These topics are from the novel “The Giver”, written by Louis Lowry in 1993.