Frosts invokes two possible meanings: green is both the color we associate with nature, such as in grass or leaves, and it also means "new." By using a word like green, with concrete visual associations, we can see green grass and foliage. We can see that in dawn's earliest sunlight (or, metaphorically, in Paradise before the Fall) grass and foliage might look golden.

In the second line, Frost uses touch imagery: "Her hardest hue to hold." Frost again draws upon two different meanings. On one level, Frost employs a concrete image: gold is physically hard. Therefore, we can understand with our sense of touch that gold is hard when we feel it. But Frost is also saying that "gold," the highest standard of life, as implied in the term "golden age," is hard (difficult) to maintain.

Other images Frost uses include "flower," "leaf," and "dawn," all things we can visualize.

Sensory images often stick in our memory longer than abstract ideas, so Frost's concretizing of the abstract concept that paradise quickly fades helps us remember this poem.

Robert Frost's, "Nothing Gold Can Stay" uses imagery to describe different states of nature. He starts out with a metaphor saying, "Nature's first green is gold." As the seasons change from spring to summer the reader is encouraged to see the leaves change from their first color that he describes as gold but that state does not last long before the leaves turn green. Frost continues his use of imagery when he speaks of Eden's grief, the reader feels the sadness associated with the story of the Garden of Eden. In the end, he describes how the earliest rays of sun are golden at dawn changing their hue to the sunshine of the day. He describes it as "dawn goes down to day," when most would speak of sunrise. The reader can see the early golden sun rays tone down to daylight as but he reminds us, "that nothing gold can stay."

"Nothing Gold Can Stay," by Robert Frost, is a poem about the illusory nature of life. This theme, that nothing of value ("nothing gold") will last forever, is substantiated through the imagery of the poem.

The title and last line are the same, and the poem can best be read as a metaphor, since none of the images in between represent actual gold. Each of the primary images of this Frost poem are "gold" for a time but eventually fade to nothingness or death. 

The first reference is to the gold found in nature, leaves which are green and gold but eventually (after only a figurative hour or so) they die.

Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf,

The same is true of the garden of Eden, the shining gold standard of perfection. This biblical allusion refers to the perfect place of God's creation which only stayed perfect for a time, until sin was introduced and death became a reality. 

Then leaf subsides to leaf,
So Eden sank to grief,

The final image is a gold sunrise sinking into a gold sunset as another day fades away (dies). 

So dawn goes down to day
Nothing gold can stay. 

Frost's theme is that everything, even the most wonderful things, will eventually fade and die. This is true of so many wonderful things in everyone's life which are here and then gone. This transience is demonstrated by the three images of this poem: leaves, the garden of Eden, and sunrises/sunsets.

The original question was edited.  I think that the strongest  emotional response to the poem exists in how there is a permanent sense of change in consciousness.  There is a clear demonstration of how being in the world is subject to change.  While one might wish for permanence and a sense of lasting in the world, the poem triggers the experience of temporality within the reader.  An emotional approach to this might depend on the reader, themselves.  In typical Frost fashion, optimists will point to how change is a part of being and representative of the greatness in life.  That which is bad will change and the continual evolution in being is what makes life worth living.  An equally valid emotional experience can emerge from the pessimist who can see the poem as a clarion call to reflect that nothing good will stay and that life is rooted only in that which passes.  Each emotional experience is a valid one.  I think that the poem speaks to both experiences in the images employed.  These images such as the dawn's movement through day, the mutability of nature, and the sinking from the Garden of Eden are all reflective of this.  There is an emotional response of reflection or rumination, in which the individual's true nature or their own emotional construction is evident.

Frost is using the changing colors of the leaves to meditate on the continuing and unstoppable changes of life.

Youth is presented as the golden age, the best of all times of life. "Nature's first green is gold" - when life first emerges, it is at its best, bright with light and value. However, this "flower" of early life lasts briefly - "only so an hour." After that first glorious emergence of life, one stage "subsides" to the next, and even when the stages don't appear to hold tremendous differences from previous ones, time is marching on. There is nothing youth can do that would allow it to remain youthful forever.

Just as "Eden," the Garden of Paradise when the world was first created, "sank to grief" when sin entered the world and Adam and Eve were evicted from the garden, so too life is destined to move on as "dawn goes down to day." It is impossible to remain in the wonderful world of youth. "Nothing gold can stay."

The essential theme of "Nothing Gold Can Stay" is that the beauty in everything fades as it ages. The initial beauty of a flower is only "so an hour." After that initial flowering, the life of the plant becomes mature, but less beautiful. Frost begins the poem stating "Nature's first green is gold," which sets it up for the remainder of his statement. It's "her hardest hue to hold" onto during life. Perhaps the best image that can help to explain this is in the second to last line: "So dawn goes down to day." At dawn, the sky can be golden and beautiful. It is something to stop and watch. But the dawn only lasts for an hour or so, before everything looks about the same for the rest of the day, until dusk, which again is beautiful, but only lasts for a little while.

As far as allusions go, Frost mentions the Garden of Eden, where Adam and Eve lost their innocence when Eve took a bite of the forbidden apple. As such, Frost lends the sense of a loss of innocence to his poem. Each morning, the dawn loses its innocence to the events of the day and Spring's flowers and leaf buds lose their innocence to the events of the summer.

Frost's poem is profound in how it shows existence as something that is mutable.  The only permanent element in consciousness is that there is change.  There is something profound in this.  Frost is able to offer a glimpse into this condition of being through a few lines that stick in the mind's eye.  For example, "Her hardest hue to hold" in reference to the opening image of "Nature's first green is to gold."  The idea that beauty, the morning pristine vision, is something that is "hardest to hold" even for the natural world is quite profound.  Frost seeks to bring out in the opening two lines that even nature cannot hold on to that which seems perfectly beautiful.  The most tender and wonderful moments in our being are transitory ones.  The happiness we experience in these instances are "the hardest hue to hold."  We are compelled to see that what nature has to endure, we do too.  Frost's poem does not make the reader entirely sad about being in the world, but rather forces one to accept that we must treasure our happiness, and not take it for granted as it can leave as easy as it arrives.

The subsequent images do much to bring this picture of being into focus.  "Eden" sinking to "grief" is another such image that forces the reader to understand the transitory nature in being in the world.  The idyllic garden is one that is in passing.  Even the creation of the divine, the realm where all is good and right, sinks according to the weight of time.  Frost's poem is a reminder that our lives are lived from the hopeful joyful experience of one instant that passes to another lying in wait. We swing from vine to vine of happiness moment to another moment of joy.  The idea that "nothing gold can stay" reminds us of just as the condition of nature and the creation of the divine, we, too, are bound to only enjoy what we can when we can.  It is not permanent.  Our happiness is elusive as it passes our grasp and Frost's poem is a reminder to revel in it because "nothing gold can stay."