An artist commissioned years ago to paint a portrait of former President Bill Clinton has revealed a secret reference -- to Monica -- tucked into the shadows of the official painting.

Pennsylvania artist Nelson Shanks told The Philadelphia Daily News that he painted in the shadow of a blue dress in the 2006 portrait -- a reminder of the political sex scandal between Clinton and former White House intern Monica Lewinsky.

"The reality is he's probably the most famous liar of all time. He and his administration did some very good things, of course, but I could never get this Monica thing completely out of my mind and it is subtly incorporated in the painting," Shanks told the Daily News.

It's definitely subtle, but Shanks pointed out that the dress can be seen casting a shadow on the mantle in the portrait. 

"If you look at the left-hand side of it, there's a mantle in the Oval Office and I put a shadow coming into the painting and it does two things," the painter told the newspaper. "It actually literally represents a shadow from a blue dress that I had on a mannequin, that I had there while I was painting it, but not when he was there." 

Shanks said the secret shade also is "a bit of a metaphor in that it represents a shadow on the office he held, or on him."

The Lewinsky dalliance led to the impeachment of Clinton in 1998 by the U.S. House of Representatives, followed by his eventual acquittal on impeachment charges of perjury and obstruction of justice in a 21-day Senate trial.

The presidential portrait first raised eyebrows when it was unveiled nine years ago, for what Shanks left out of it: Clinton's wedding ring. The revelation about the dress adds another intriguing detail to the history of the portrait.

The Clinton oil painting is among a collection of 55 of the 42nd president that the National Portrait Gallery has in its possession, Bethany Bentley, a spokeswoman for the gallery, told FoxNews.com.

In the Daily News interview, Shanks claimed the "Clintons hate the portrait" and are putting "pressure" on the gallery to have it removed. Bentley denied this.

"No, they have not asked us [to take it down]," she said.

She also said the painting in question has not been on display for a few years.

The NPG has the nation's only complete collection of presidential portraits outside the White House. The "America's Presidents" exhibition includes oil paintings, sculptures, photographs and caricatures of each president.

Bentley said the gallery, when overseeing a portrait, presents a "variety of artists" and works with the subjects to determine who would best match up. The field is then narrowed to artist availability and timing. The NPG does not ask about an artist's political leanings.