Sri Lankan vs Asian

Sri Lankan and Asian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sri Lankan	Yes	Yes	No	No
Asian	Yes	Yes	No	No

As nouns, Asian is a hypernym of Sri Lankan; that is, Asian is a word with a broader meaning than Sri Lankan:

Other hypernyms of Sri Lankan include Asiatic.

Sri Lankan (noun)	Asian (noun)
a native or inhabitant of Sri Lanka	a native or inhabitant of Asia

Sri Lankan (adjective)	Asian (adjective)
of or relating to Sri Lanka (formerly Ceylon) or its people or culture	of or relating to or characteristic of Asia or the peoples of Asia or their languages or culture