Johnny: "Stay gold."
Earlier in the book, we learn that nothing gold can stay. Meaning if something good is happening, it won't stay good forever. But in this situation, Johnny is saying this because he wants Ponyboy to stay a good person. He wants Ponyboy to stay like he is, perfect. He wants Ponyboy to keep doing what he is doing. Helping society. And maybe that will help Greasers and the Socs become one with each other