Golden Globe nominee Scarlett, 27, blossomed in a black and scarlet floral corset and lace skirt by Prada with bright red heels, while Canadian actress Cobie, 30, lived up to her surname in a red creation by New York designer Alexandra Vidal.
The actresses, who play Russian spy Natasha Romanoff and S.H.I.E.L.D agent Maria Hill respectively, were joined by co-stars Robert Downey Jr (Iron Man), Tom Hiddleston (villain Loki), Mark Ruffalo (Hulk), Jeremy Renner (Hawkeye) and Clark Gregg (Agent Phil Coulson) at the premiere of the Marvel superhero blockbuster at Westfield London.
Scarlett said she enjoyed reprising the role of the feisty Black Widow, originally seen in Iron Man 2, despite the injuries she suffered doing her own stunts.
"It's nice to be able to play a female character that's not a damsel in distress or some woman in waiting but to actually play someone who's got a lot of conviction and who is really ready to kick some booty," she said. "It was just all one big injury the whole thing - you get used to it I guess."
Cobie, who also stars in hit US series How I Met Your Mother, added: "It's been a dream role for me. I love it."
Robert said it was "a blast" to be in London, saying: "It's been five years since our last adventure so I'm happy and humbled to be here."