What does the metaphor in Johnny's letter mean when he asks Pony to stay gold?
What Johnny is saying to Pony is that he does not want him to change. He does not want Pony to become hardened and bitter. He thinks that Pony has the potential to become something and he does not want him to end up like, for example, Dallas Winston.
In the poem, Frost says that things that are beautiful (like gold) cannot stay that way. He says that the first leaves that come out are the most beautiful, but they too cannot stay that way. Johnny is trying to encourage Pony to resist that trend and to stay beautiful (inwardly) and good.
In the book "The Outsiders" I recall when the two were in the church and talking. Johnny was puzzled by the poem and the wonderment of it. Johnny had rescued the children with Pony Boy and he related the poem to the children and Pony Boy.
When children are young they are innocent to the evils of the world. They see the good in things and the joy in life. For Johnny and many of the greasers life had dealt them hard blows. Dally had become angry and bitter. The others fought their anger and contempt away. Pony Boy was different. He had innocence to him.
Johnny wanted Pony Boy to stay innocent and to still recognize the things of beauty in the world. He wanted him to enjoy life and live like a child. Stay golden, stay innocent.