In simple words, Season's "first green" flowers are connected to golden, earth's most valuable metal, instantly cementing gold as a metaphor of anything that is new, young, and lovely.
The following line, "Her hardest hue to hold," indicates that maintaining the innocent of the initial greens is the most difficult things to accomplish and he adds, "Her first leaf's a blossom / only so an evening." This is the third time he uses an analogy, suggesting that a blade is a bloom (and green is gold).
