请专业人事看一下

dress up in sth 固定词组 化妆打扮的意思
例如：they were dressed up in Victorian clothes.
他们化装成维多利亚时代的人。
(直译：他们穿上维多利亚时代的衣服)

dress up in costumes 穿上（万圣节）服装

dress up 1.打扮2.穿上盛装
这里指put on one's best clothes so as to look pleasantly,此处为第二个意思。
in costumes 穿着（特定）服装。

dress up - make up/ wear something appropriate for the party

costumes- dress ups like e.g a vampire/ a tv character etc

万圣节前夕，人们盛装出席party。
dress up解释为盛装、打扮
in costumes解释为穿节日服装，在整句翻译里可以不译

in costumes 穿着戏装,就是不是平常穿的便装
dress up 收拾打扮,穿上合适的服装