Employed with great artistry, color imagery underscores the symbolism in Fitzgerald's The Great Gatsby. At the end of Chapter One, for instance, Gatsby is seen by the novel's narrator, Nick Carraway as he stands trembling with arms extended gazing at the green light of hope and renewal at the end of Daisy Buchanan's pier. Then, in Chapter Two, with ironic contrast the Valley of Ashes is called "a fantastic farm" with "grotesque gardens"; thus, the green of renewal is tainted with the waste of ashy grey land and "the spasms of bleak dust which drift endlessly over...the solemn dumping ground."
Against the wasteland of Chapter Two, Gatsby's green dream of regaining Daisy soon becomes greyed and made insignificant and empty. Likewise, some of the characters in the novel are belittled and unnoticed through the use of grey imagery. George Wilson, for example,"mingl[es] immediately with the cement color of the walls" of his garage. And, James Gatz is described in Chapter Six as a "grey, florid man with a hard empty face," and the amoral Jordan Baker has grey eyes.
The color grey describes the degradation of the American dream, its falsity built upon the acquisition of wealth. In Chapter Eight from which the example quote comes, Gatsby's house acquires an "inexplicable amount of dust" after the death of Myrtle Wilson. The dawn fills the house with "grey turning, gold turning light. The shadow of a tree fell abruptly...." while in the Valley of Ashes the distraught George Wilson stands "nodding into the twilight." These occurrences presage the doom of Gatsby's dream.
Wilson's glazed eyes turned out to the ashheaps, where small grey clouds took on fantastic shapes and scurried here and there in the faint dawn wind.
Wilson's deranged and wasted thoughts take on "fantastic shapes" that recall the ashheaps of the valley; like them his thoughts are distortions of what is real and good. And, he sets forth later on his grotesque deed, making Jay Gatsby a tragic hero.
In F. Scott Fitzgerald's The Great Gatsby, literary devices such as color imagery merge to reinforce and clarify motifs and the theme that Americans have wasted the original American Dream by becoming materialistic and amoral. The materialism of the East generates the Valley of Ashes, the grey destruction in which values die and "scurr[y] here and there in the faint dawn wind."
The "gray clouds" you cite are clouds of ash dust blowing off the ashheaps in the valley of ashes. The hopelessness and helplessness in which the inhabitants of that area live clings to them just like the dust.
Formed into "fantastic shapes," the clouds can appear to be signs of hope and better times ahead. George Wilson was hoping to get Myrtle and himself away, to build a new life for themselves somewhere far from Myrtle's affair with the wealthy client. Of course, George didn't know that Myrtle was in love with Tom Buchanan, who was so condescending toward George in his role as garage owner.
By the end of the chapter, George himself becomes an "ashen, fantastic figure" moving toward Gatsby, covered in the ashes and dust of his destroyed home, approaching to take the matter of vengence into his own hands.