"I didn't believe it," she tells PEOPLE. "Then I had a fitting and I wore black granny panties on purpose, and sure enough, I put on the pants, I bent over in front of the mirror, and I screamed to my assistant, 'You can't see the underwear!'"
The mother of three is the face of Spanx's brand new Silver Linings technology, the first-ever collection of all-white shorts and pants that boasts "100% opacity" no matter the undergarment. To celebrate the technology that was seven years in the making, Spanx partnered with Graham, 34, who calls the shapewear empire "iconic."
In a billboard unveiled in Los Angeles Tuesday, Graham bares it all, posing fully nude, covered only by the Silver Linings fabric. The fabric successfully covers the entirety of the supermodel's nude figure, demonstrating the effectiveness of the material.
For the Lincoln, Nebraska native, this billboard is a first. "I don't think I've ever been naked on a billboard before," she tells PEOPLE. "I'm trying to remember — I've been modeling for so long — but I don't think I've ever been naked."
The partnership with Spanx is a whole host of firsts for Graham, who welcomed twin boys, Roman and Malachi, with her husband Justin Ervin in January. It marks her first time posing naked for a billboard, but is also one of the first projects she's worked on since giving birth.
"I'm taking it easy," she shares. "I'm slowly getting back into work, and I'm picking and choosing very carefully. I don't want to feel overwhelmed rushing back into work."
Graham and Ervin are also parents to son Isaac, 2, and she calls having three kids all under the age of three a "full-time job."
"6am to 9am is already a full day," she jokes with PEOPLE.
In transitioning back to working, the Sports Illustrated Swimsuit cover star and runway model — who is known for her body positivity and openness on social media — says there was definitely a period of adjustment.
"My body has changed so much," she tells PEOPLE. "I have much more gratitude toward my body because of how much work it's been through, but it has definitely taken time to, you know, look at my new body, and my postpartum body, and recognize it again."
She shot the Spanx billboard just three and a half months after giving birth, and said she was able to feel confident going in because she "felt good mentally." Her typical pre-photoshoot prep involves practicing affirmations. Before the Spanx shoot, Graham "had a little talk with [herself] in the mirror," she tells PEOPLE. "I said, 'Everything's gonna be okay. You look fantastic.'"
The innate confidence that Graham possesses is part of what made her the perfect partner for the latest Spanx launch. "We are thrilled to partner with Ashley on this collection because she exudes confidence and ease, something we could all use more of," says Spanx President Kim Jones in a press release.
Available in 4" and 6" Shorts, Kick Flare and Ankle Slim Straight pants, Spanx calls the new collection "the first pair of white pants you can underthink." Noting the stress that can come with donning white pants — "worrying about if someone can see their underwear, where they can and can't sit" — the new technology alleviates the feeling of all-day overthinking, and instead lets women feel "powerful in their own skin and clothing."
Each style is stretchy, with a pull-on design, and hidden "tummy shaping" technology for comfort and confidence.
The technology is "durable," Graham says. "You don't have to worry about your underwear, or what time of the month it is, or what you're eating that day. I mean, what a novelty."
Even as a mother dealing with "throw up, poop and dirt all over me," Graham says the new Spanx piece fit in seamlessly to her busy mom routine, noting that the line keeps the worry out of her wardrobe.
"They're the kind of pants that feel accessible to whatever I'm doing in my life," she tells PEOPLE.
The supermodel's brave new billboard is now on display in Los Angeles, and the Spanx On The Go Collection is now available at spanx.com.