Answer:
This is one of my favorite poems. Also referenced in The Outsiders.
This poem, "Nothing Gold Can Stay" is talking about a sunrise. The sky is gold in the morning when daytime first starts. "Her hardest hue to hold," is talking about the color of the sunrise and how it only lasts "only so an hour." It's talking about a sunrise, but the theme it's trying to get across is nothing gold can stay, or good moments in time can't last forever.