。
She was wearing a green dress.
Literal
She [topic-は] green [of-の] dress [object-を] was-wearing.
Color-name choice is interesting here: グリーン (the loanword from English) coexists with the native 緑 ('green'). The loanword tends to appear in specifically fashion, design, and product contexts where it sounds more stylish or modern — グリーンのドレス sounds contemporary and Western, while 緑のドレス would sound slightly more plain. The same split exists across many colors: ピンク vs 桃色, ブラウン vs 茶色, ブルー vs 青. ドレス is also a loanword, specifically meaning a Western-style dress (as opposed to native 着物 / 和服). The whole phrase places the wearer in a Western-fashion frame.