white beer vs witbier

white beer and witbier both are nouns.

Word	Noun	Adjective	Verb	Adverb
white beer	Yes	No	No	No
witbier	Yes	No	No	No

As nouns, witbier is a hyponym of white beer; that is, witbier is a word with a more specific, narrower meaning than white beer:

white beer: a general name for beers made from wheat by top fermentation; usually very pale and cloudy and effervescent
witbier: a top-fermented barley/wheat beer brewed mainly in Belgium and the Netherlands.

Other hyponyms of white beer include Weizenbier, Weizenbock.

white beer (noun)	witbier (noun)
a general name for beers made from wheat by top fermentation; usually very pale and cloudy and effervescent	a top-fermented barley/wheat beer brewed mainly in Belgium and the Netherlands.