。
She is wearing a green dress.
Literal
She [topic-は] green [possessive-の] dress [object-を] is-wearing.
緑色 (みどりいろ, 'green color') uses the ~色 suffix for color specification. 着ている (きている) is the standard way to express wearing clothes — the resulting state of having put them on. ドレス is a loanword from English.