fashionable vs in fashion

fashionable and in fashion both are adjectives.

Word	Noun	Adjective	Verb	Adverb
fashionable	No	Yes	No	No
in fashion	No	Yes	No	No

As adjectives, fashionable and in fashion are synonyms defined as:

fashionable and in fashion: popular and considered appealing or fashionable at the time

fashionable (adjective)	in fashion (adjective)
being or in accordance with current social fashions	popular and considered appealing or fashionable at the time
having elegance or taste or refinement in manners or dress
popular and considered appealing or fashionable at the time