costumier vs couturier

costumier and couturier both are nouns.

Word	Noun	Adjective	Verb	Adverb
costumier	Yes	No	No	No
couturier	Yes	No	No	No

As nouns, couturier is a hypernym of costumier; that is, couturier is a word with a broader meaning than costumier:

costumier: someone who designs or supplies costumes (as for a play or masquerade)
couturier: someone who designs clothing

Other hypernyms of costumier include clothes designer, designer, fashion designer.

costumier (noun)	couturier (noun)
someone who designs or supplies costumes (as for a play or masquerade)	someone who designs clothing