To understand this, we need to look at what colors often symbolize in literature.  Here are some meanings colors have that go beyond what we see:

White—purity, innocence, goodness, light

Red—passion, love, anger

Green—envy, freshness or newness

Yellow—cowardice

Blue—sadness, royalty

Black—evil, mystery, secrecy, death, darkness, power, the unknown

Reverend Hooper’s black veil symbolizes the sins he is hiding from the community.  It covers and hides his eyes so no one can see into his soul and understand the sins he has committed.  The black veil is mysterious to the town’s citizens, as they don’t understand the strange appearance of Hooper.  Hooper is keeping a dark secret, a secret that is founded in evil doing and sin.  The speculation is that Hooper’s sin involves a young woman in town; however, the veil is also a symbol of everyone’s sin, the “secret sin” he preaches about in church the day he first wears the veil. 

If the veil had been a different color, the entire story would have been different.  The color white would completely change Hooper’s intentions to point out the similarities he has with the other town people who have sinned.  White would indicate that he was sin free, pure, and good.  Veils of other colors would also mean different things for the reader.  Green—What does he envy?  Blue—Why is Hooper sad?

So, each color would symbolize something else.  It’s important for the veil to be black so Hooper can outwardly show his digressions and so Hawthorne can get across a message about how we all sin.