The British singer displayed the transformation — which featured bold bangs and a warm copper hue — at an event on Thursday to celebrate the launch of filmmaker Sofia Coppola's new book.
As the event was in collaboration with French fashion house Chanel, Allen, 38, fittingly wore a black patent jacket with matching silk shorts from the designer.
She attended the glitzy soirée with husband of three years David Harbour, who wore a dark suit over a gray button-up shirt.
The "Smile" singer shared some snippets of the glamorous party via her Instagram Story, including a snap of her elegant place setting and the evening's menu, as well as one that showed a "Chanel Bingo" game played during the evening.
"I did not win," Allen captioned the latter photo.
Allen had unveiled her hairstyle ahead of the event via her Instagram Story on Thursday.
The "Not Fair" singer also tagged the team behind her new look, revealing that hairdresser Ward was responsible for her hair while makeup artist Amrita Mehta applied her signature flicked eyeliner.
Allen — who shares daughters Ethel, 11 and Marnie, 10 with ex-husband Sam Cooper — is known to change up her style quite frequently. In February, she dyed her naturally dark locks blonde, and has recently been showing off a peachy hued cropped style.
Earlier in September, Allen paid tribute to husband Harbour, 48, in honor of their anniversary. She shared a snap of their low-key Vegas wedding, and wrote alongside it, “3 years with this guy, 2nd best decision I ever made. 1st was that dress.”
The photograph showed the smiling pair on a Vegas street, with Allen — who at that time had dark hair and short bangs — wearing a short Dior dress and veil while carrying a colorful bouquet.
Allen and Harbour tied the knot in Las Vegas during a low-key ceremony officiated by an Elvis Presley impersonator — followed by In-N-Out burgers!
"We can share this experience of walking through life together," Harbour continued. "I'm always so overcome with gratitude for that, because I've been alone for most of my life."
"She really is just a deeply kind individual and I love that about her," the actor added.