If you read that section of Chapter 5 carefully, you will notice that Gene does not simply put on Finney's pink shirt, but that he puts on all of his clothes. The pink shirt is the final piece of clothing he dons.
Now, think about all the expressions we use about people's clothing. For example, we say, "The clothes make the man," or "Try walking a mile in his shoes." We also make reference to a "wolf in sheep's clothing." All of these are metaphors for the idea that we are what we wear. Of course, that is not exactly true all of the time, but don't you feel like a different person when you get all dressed up for a fancy event?
Let's get back to Gene and Finney now. Finney's leg is "shattered," and Gene is responsible for this because of what he did on the tree branch. What are Gene's feelings about Finney? His feelings about Finney are quite complicated. He admires Finney, and he envies him, too. He admires Finney so much he would like to be Finney, and his envy makes him want to be Finney, too. At the very least, he would like to know what it feels like to be Finney, since Finney has so many qualities he does not have. These are some of the emotions that Gene is experiencing as he puts on Finney's clothes. If he puts on the clothes, perhaps he will be Finney. And he tells us, "I was Phineas, Phineas to the life" (62). The other emotion Gene is experiencing, of course, is guilt because he is responsible for Finney's fall. Perhaps, if he can "become" Finney, he can replace him and this will atone for what he has done.
So, you see, like all human beings, Gene has complicated emotions that cause him to do things we might consider odd if we did not know the whole story. Sometimes we do not even understand ourselves why we do things. But as we read this novel, we can see the "big picture" and understand why Gene acts as he does.