Dress up in强调的是盛装打扮，看这个例句: its halloween . children dress up in costumes at night万圣节前夕。晚上孩子们盛装打扮。
be dressed in：是穿着的含义，但是其表示的是一种穿着某种衣服的状态。
例如： I am dressed in a red coat. 我穿了一件红色外套（重点在表达穿着红外套的这个状态，而不是“穿”这个动作）。
dress up in穿着……作装扮，
例：
—Why is she dressed in a red gown？ 她为什么穿着红袍？
—Because she wants to dress up as Santa Claus. 因为她想打扮成圣诞老人
dress in 穿着 （使）穿……的服装
例：dress in a suit 穿着正装
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.
