Amanda, 42, who vamped up her appearance with bright red lipstick and snakeskin Louboutin heels, later revealed that her style may have been inspired by Sex and the City character Samantha Jones, tweeting, "Homage to Samantha Jones? #satc."
Her fellow judge Alesha Dixon, who welcomed daughter Azura Sienna in October, also chose a show-stopping look. She stepped out in a burnt orange ensemble, featuring a prom-style voluminous skirt, matching long-sleeved top and waist belt.
It was business as usual for the boys, meanwhile; Simon Cowell opted for a pair of his trademark jeans and a jumper, while David Walliams looked smart in a pinstripe suit.
So far, BGT auditions have been held in Belfast and Birmingham, with the show set to travel to Manchester and London next month.
Earlier this month, Amanda shared a photo of the judges travelling to Northern Ireland in Simon's private jet.
Taking to Twitter, she joked, "On my way to board Cowell Air and fly to Belfast! …'What you flying in?' Holding on to Simon's cape!"