Attire is uncountable, see http://www.ldoceonline.com/dictionary/attire
Not a teacher.
-John couldn't have been mistaken for Peter. Their attire(s) is/are different. The murderer must be...
We are talking about two persons/things, so is the plural form optional and I can use the singular or the plural as I like?
"Clothes" or "clothing" is more natural.
Thank you, Dave. But do you think ATTIRE could be both plural and singular?
Do you think, as you suggest, this is OK:
-Their clothing is/was different
'Their clothing is/was different.'
'Their clothes are/were different.'
Do native speakers think ATTIRE could be both plural and singular?