As a final note marking Hester's grave, this line does indeed reveal the contrasts in the text and summarize the entire novel. In order to understand how, it is important to understand the meaning of the words in the epitaph.
Field, here, means background. Sable is black and gules is scarlet. Essentially, this epitaph reads: "On a black background, a red letter A."
At face value, Hester is laid to rest still bearing the mark of her sin that she once bore in life. However, on a symbolic level, this epitaph reveals so much more. The black background in the minds of the Puritans was Hester's black heart, or evidence of sin. The red burned upon it, almost glowing, as if ADULTERY was the marring device in her life and the lives of others.
In this town and society, however, the black background could very well symbolize the hypocrisy and judgement of everyone else. The red letter A, burning in the foreground, could symbolize Hester's ability to rise above the label she was given, and instead of becoming the symbol of shame and dishonor that she wore, it would symbolize everything she was able to overcome.
There is a contrast of colors. But there is also a contrast of a societal expectation and the simultaneous hiding of every other sin, because Hester Prynne certainly was not the only sinner in her town during her lifetime. The entire novel is Hawthorne's message about such hypocrisy, and in the same way that Hester's epitaph is permanent, so too is Hawthorne's message.