yellow flag vs yellow water flag

yellow flag and yellow water flag both are nouns.

Word	Noun	Adjective	Verb	Adverb
yellow flag	Yes	No	No	No
yellow water flag	Yes	No	No	No

As nouns, yellow flag and yellow water flag are synonyms defined as:

yellow flag and yellow water flag: common yellow-flowered iris of Europe and North Africa, naturalized in United States and often cultivated

Other synonyms of yellow flag include Iris pseudacorus, yellow iris.

yellow flag (noun)	yellow water flag (noun)
common yellow-flowered iris of Europe and North Africa, naturalized in United States and often cultivated	common yellow-flowered iris of Europe and North Africa, naturalized in United States and often cultivated