dress in
(使)穿 ... 的服装
You should dress in a way that befits a woman of your position.
The bride may dress in red silk and may wear cardboard wings with feathers on her head.
dress up in
穿上
They're all dress up in their robes and their bed sheets and act like a bunch of ghosts or spooks or something.
俩者都表示“穿”,dress up 更侧重与穿的动作和状态；dress in 强调服装的样式
dress up后加人,表示给谁穿衣服
例句：She is too young to dress up herself.
