What do you think Johnny means when he tells Ponyboy to "stay gold"?

Johnny Cade utters this last phrase, likely one of the most famous lines in all of The Outsiders, to Ponyboy as he lies dying. The quote is a reference to the line "Nothing gold can stay" from the Robert Frost poem that Ponyboy recited to Johnny when the two were hiding at the church. This notion is applied to the concept of youthful innocence throughout the novel, and Johnny is begging Ponyboy not to grow bitter and cynical and to retain his childlike sense of wonder about the world. The quote is also a reference to the time at the church, where the two could temporarily forget about the hardships of their lives outside that particular interlude. It was a part of their lives that made their friendship into a brotherhood. That was the "gold" that couldn't stay. Johnny is asking Ponyboy to be an exception to the rule, by being something gold that stays.

After the rumble, Dallas and Ponyboy go to the hospital to see Johnny. When they arrive, Johnny is still and quiet. During their visit, Johnny tells Ponyboy to "Stay gold." This comment is in reference to an earlier event in the story. In chapter 5, Johnny and Ponyboy observe a beautiful sunrise. Johnny comments on the mist, "All gold and silver." Ponyboy then quotes a line from a Robert Frost poem in his reply to Johnny by stating that "Nothing gold can stay." He remembers the line but is unsure of the poet's intent.
After Johnny's death, Pony finds a letter inside the copy of Gone With the Wind that Johnny had in the hospital. By reading the letter, Pony understands why Johnny told him to "Stay gold." Johnny explains that being gold is like being a kid, when "everything's new." He compares Pony's love of sunsets to being gold and instructs Pony to have Dallas look at a sunset. Johnny's words help Pony understand that Johnny wants him to see life through a youthful and positive perspective.