Hi Alan! I've never heard people say 化妆服 colloquially, but that can be a difference between China and Taiwan. We say 服装, which means outfit in general, but also means costume. Another way of saying costume is 戏服(For actors/actresses), 表演服装(For dancers), and 道具服(For performers), depending on the context. Generally, we say 服装. 1. What costume are you wearing for this year's comic con? 今年动漫展你穿什么服装？ 2. I'm going to put on my Halloween costume now. 我现在要穿上我的万圣节服装. 加油！

I would use 衣服 to address clothings in everyday situation. 服装 is actually very casual as well. 今年comic con你穿什么衣服？ 我要穿我的万圣节衣服。

Chou's answer is pretty good. I'm afraid there is not an equivalent for COSTUME (in the meaning of ' An outfit or a disguise worn on Mardi Gras, Halloween, or similar occasions' in spoken modern Chinese__ if any, 行头 may be one , but it's rarely used today except in some limited rigions( Beijing, for one), and this word is used with a jocular tone__ When you have to say' What costume are you wearing for this year's comic con? ' and ' I'm going to put on my Halloween costume now' in Chinese, you may possibly otherwise say '.... 打算穿什么（衣服）, or more possibly '打算怎么打扮' and ' ...穿上要在Halloween穿的那身打扮‘ or ’ ...穿上要在Halloween上穿的衣服了( it may sound somewhat clumzy, but it's the case when you have to put something in another language which has not an equivalent for it‘.