What role does the color gray play in The Great Gatsby?

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."

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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.

What roles does the color blue play in The Great Gatsby?

Suggesting both color, mood, and music, blue has several meanings in The Great Gatsby.

THE COLOR BLUE

• A color that connotes dreams and illusions, Jay Gatsby shows Daisy his many colored shirts that monogrammed in "Indian blue" His gardens indicate the hidden boundary of Gatsby's world from that of the real one as his gardens are described as blue in Chapter Three:

In his blue gardens men and girls came and went like moths among the whisperings and the champagne and the stars.

• Likewise, the chauffeur who has invited Nick to Gatsby's house wears blue. In Chapter Six after a reporter inquires about Gatsby, Nick Carraway launches into a history of James Gatz of North Dakota, whose mentor, Dan Cody, takes him to Duluth and buys him "a blue coat, six pair of white duck trousers and a yachting cap." Nick describes the young Gatsby as having "fantastic conceits":

For a while these reveries provided an outlet for his imagination; they were a satisfactory hint of the unreality of reality, a promise that the rock of the world was founded securely on a fairy's wing.

• Later in Chapter Six, Daisy inquires about the name of a man with a "blue nose," suggesting further illusion. Then, Gatsby tells Nick that he is going to get Daisy back as she will divorce Tom and come with him.  When Nick argues that no one can repeat the past, Gatsby counters, "Can't repeat the past?...Why of course you can!" 
• In Chapter Seven after Myrtle is struck by Gatsby's car that Daisy has driven, the police question Tom Buchanan, asking him the color of his car. "It's a blue car, a coupe," he replies.
• In Chapter Eight Nick and Gatsby enter his house as the "birds began to sing among the blue leaves." There is a gloom now to his illusions as Gatsby remarks, "I don't think she ever loved him." And Nick reflects,

What could you make of that, except to suspect some intensity in his conception of the affair that couldn't be measured?

• Later, Nick returns to Wilson's house where he finds Michaelis staying up through the night with the shaken husband. Michaelis is relieved when he notices a change in the room,

a blue quickening by the window and realized that dawn wasn't far off. About five o'clock it was blue enough outside to snap off the light.

• Just then Doctor T.J. Eckleburg emerges with pale eyes in the dawn, and Wison repeats, "God sees everything."

BLUE AS A MOOD

• In the artificial world of silver and golden slippers on dance floors that Daisy has grown up, 

All night the saxophones wailed the hopeless comment of the Beale Street Blues

• At Gatsby's party, she sings to "Three O'Clock in the Morning," a "neat sad little waltz of that year."
• As Gatsby carries his mattress and heads for his pool in Chapter Eight, his gardens and leaves are no longer blue, but are now the color of decay; Gatsby

shook his head and in a moment disappeared among the yellowing trees.

The color and music of blues indicate Gatsby's dreams and illusions; yellow signifies that they have been degraded by the dissolution of his idealization of Daisy.

What roles does the color white play in The Great Gatsby?

Interestingly, in The Great Gatsby, Daisy's name carries signficance regarding the color white.  Just as her clothing and her car years ago when she first met Jay Gatsby were white and suggested innocence, the flower after whom Mrs. Buchanan is named deceptively suggests a certain naivete and dreaminess.  However, the deception lies on the exterior, for the center of the flower is gold, and as Nick observes, Daisy's voice is "full of money"--

that was in inexhaustible charm that rose and fell in it, the jingle of it, the cymbals' song of it....High in a white palace the kings' daughter, the golden girl....

So, despite her airy white dreaminess that enjoys the illusion of love, Daisy thrives on wealth.  This is why she married Tom Buchanan; this is why she will not leave him for Jay Gatsby and why she hides behind her husband's wealth and power after she kills Mrytle Wilson. Daisy's spiritual veneer is a gossamer and unattainable white, an airy ghost of a real soul because she is the "bad driver" of whom Nick metaphorically speaks, the type of person whose lack consideration of the impact of her actions upon others near her wreaks tragic results.

White is the color of innocence, the color of purity. When Nick first sees Daisy and Jordan, they appear almost angelic in white dresses that "were rippling and fluttering as if they had just been blown back in after a short flight around the house." As Nick (and the reader) come to realize, people and things are not always as they appear.

Nick moved to New York to enter business. He found employment working in the bond business, and

Most of the time I worked. In the early morning the sun threw my shadow westward as I hurried down the white chasms of lower New York to the Probity Trust.

The implication is that the business was completely honest and for the benefit of all who were involved. Any hint of corruption or illegal business deals or persons more concerned with making a fortune for themselves, regardless of the impact on anyone else, is hidden.

Naturally, the eighteen-year-old Daisy lived in a white mansion, "dressed in white, and had a little white roadster." She was (or, at least, her reputation held that she was) the sweet young thing, the virgin with no knowledge or interest in anything that wasn't proper.