Answer: When Johnny said that to him, he meant for him to stay innocent. He meant for Ponyboy not to go down a bad path like many of the other greasers did. It was a reference to the Robert Frost poem "Stay Gold". Ponyboy had recited the poem to Johnny when they were hiding out at the Windrixville church. In one line of the poem, it had said "Nothing gold can stay" meaning that all good things must come to an end. So he was telling him to stay gold or stay good. So if you think about it he was telling Ponyboy not to change.
