The Duchess is wearing the Missoni Metallic Lurex Midi Dress (with thanks to Middleton Maven).
https://hrhduchesskate.blogspot.com/2019/02/first-look-kate-in-green-midi-dress-for.html
https://hrhduchesskate.blogspot.com/2019/02/first-look-kate-in-green-midi-dress-for.html