I think Frost is writing about how life is not always perfect. There are definitely wonderful, "gold" moments throughout life. They do not even have to be decadent moments to be perfect. For example, "green" appears as "gold" and a simple leaf is "a flower." It does not last - "only so an hour" before "Eden" sinks. However, dawn does not turn to night but to day. The perfect moment may have passed, the paradise may be gone, but there is still light.