Hipstaa is definitely right about the green light. Located at the end of Daisy’s dock in East Egg, it represents Gatsby’s hopes and aspirations. But green is also the color of spring and new birth; thus, it is associated with Gatsby’s hope for a new beginning with Daisy. Like a green light in traffic or a guiding light, it tells him to “go” for his dream—not only his dream for Daisy, but in more general terms the American Dream of wealth and success. It is also the color of money (gold, also associated with money, is also symbolic in this novel), and it is his great wealth that leads the once-poor Gatsby to believe he has a chance to start anew with Daisy. Thus, the symbolism of the color green takes on many nuances.
The Valley of Ashes, literally a long, dreary stretch of land between New York City and Long Island where industrial ashes from incinerators are dumped, does represent desolation and darkness. It is also important to note that it symbolizes the moral decay underlying so much of the behavior depicted in this novel. The section on the theme of "moral corruption" in the e-notes guide cited below expands on this topic, clearly associated with the Valley of Ashes. The associations are manifold, including the ashes of Ash Wednesday, reminding us of the Biblical exhortation that we are from dust and will return to dust.