Why does "white ashen dust" veil Mr. Wilson's "dark suit and his pale hair" in The Great Gatsby?
"White ashen dust" veils Mr. Wilson's "dark suit and his pale hair" because Mr. Wilson lives and works in the Valley of Ashes, a veritable wasteland inhabited by factories, railroad tracks and blue collar workers struggling to survive. Physically, the dust is pollution from the factories that settles all over the surrounding landscape. Accordingly, nothing in the Valley suggests vitality or prosperity. It is a "dead zone," and ash, dust, and pallor are three images that strongly suggest death. The Valley of Ashes is located between West Egg and Manhattan; thus, it is poised between the residences of the extravagantly wealthy and the city where these wealthy do business.
Mr. Wilson is a struggling mechanic who can barely make ends meet, and he is losing his wife to an affair with Tom Buchanan, a man from East Egg. Wilson is a man whose hope for a better life is dying, just as he is. Fitzgerald, throughout the novel, comments on Wilson's wan, "pale" appearance, which only worsens as he senses his wife slipping farther and farther from his grasp.
The "white ashen dust" on his suit also foreshadows his wife's hit-and-run death, which takes place later in the novel, right in front of his garage. In an act of mistaken revenge (he believes Gatsby has killed her, while Daisy was the one really driving), Wilson murders Gatsby and then takes his own life. Wilson is even described as an "ashen figure" as he emerges from a bush in Gatsby's lawn to murder him. Wilson then has the death of two human lives "on his hands." This death is inescapable for Wilson, just as he could not escape the ashen dust of the Valley he inhabited; the extravagantly wealthy of East Egg, however, live in their pristine neighborhood, free of dust, responsibility, and death. Daisy, the East Egger and driver in the hit-and-run, faces no consequences.
Fitzgerald uses color and light and dark as symbolically as possible.
If you make it all the way to the end of the book, you discover that Wilson believes a crime has been committed which hurts him. He's right. Someone is having an affair with his wife... he just doesn't know who.
The white ashen dust covering his dark suit represents the innocence covering his sin. His innocence is that something or someone else is wronging him. Another truth is that he isn't providing his best for his wife and he eventually commits a severe crime because of how he can't provide anything all that good for his wife.
So he is evil on the inside, but doesn't want to be. He wishes to be a good man. Not knowing where you are in the book, I don't want to give too much away.