What does it mean when Johnny tells Ponyboy "be gold" in The Outsiders?
I assume you are referring to The Outsiders. In The Outsiders, there is reference to the following poem:
Nature's first green is gold,
Her hardest hue to hold.
Her early leafs a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.
This poem is called "Nothing Gold Can Stay" by Robert Frost. The meaning of the poem is that everything starts out young and innocent, but it cannot stay. Good is "the hardest hue to hold" because there are so many influences just from living life that can corrupt us. When Johnny tells Ponyboy to "stay gold" he means that he wants him to stay good, and not be corrupted by the negative forces in the world.
The Outsiders is a coming of age story. Although Ponyboy is a good person, he gets caught up in gang life and ends up going on the run after a boy dies. Throughout the story, Ponyboy keeps his good heart and does manage to stay gold.