Here's the special meaning behind Michelle Obama's official portrait gown

Michelle Obama's official portrait was unveiled inside the Smithsonian Museum -- and like her husband's, it was far from traditional.

Painted by artist Amy Sherald, the former FLOTUS is pictured sitting in a chair wearing a white halter gown with geometric shapes. The dress was a custom creation from American designer Michelle Smith of the label Milly, based on a look from her Spring 2017 collection.

Smith knew that Mrs. Obama would be wearing one of her dresses for a portrait six months ago, but she didn't know when exactly it would be unveiled. She got the call from her publicist while she was in Paris for work.

"I was in a shop buying jeans when my PR called. I started flipping out at the cash wrap," Smith said of her initial reaction.

As for the dress, the designer revealed that it's supposed to be one "that Mrs. Obama could wear in her everyday life, as well as in this iconic portrait."

She continued: "It’s made of a stretch cotton poplin print in a clean, minimal, geometric print without a reference to anything past or nostalgic, which gives the dress a very forward-thinking sensibility -- this is very Michelle Obama."

Many fashion experts wondered why Michelle didn't opt for Michael Kors or Jason Wu -- two luxury brands that she had worn many times as first lady. However, her decision to wear Milly reflects her legacy of approachability and sensibility. She was never afraid to mix budget-friendly labels like J. Crew with her pricier designer items.

"It’s up to Mrs. Obama to say why she chose this for the portrait, but I would say that it’s a very modern, emotional dress with a very womanly, very American spirit," Smith said. “The dress also reveals her arms, which I believe is groundbreaking and very modern for a portrait of the First Lady."

Smith has met Obama several times, including the former first couple's final party in the White House last January.

"It was the best night of my life. No phones were allowed inside, so everyone let loose, was in the moment, and had a wonderful time. No one wanted to leave. I think Andy [Oshrin, her husband and business partner] and I staggered out around 4 a.m."

For more on Michelle's portrait, watch the video above.