mainland China vs Red China

mainland China and Red China both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainland China	Yes	No	No	No
Red China	Yes	No	No	No

As nouns, mainland China and Red China are synonyms defined as:

mainland China and Red China: a communist nation that covers a vast territory in eastern Asia; the most populous country in the world

Other synonyms of mainland China include Cathay, China, Communist China, People's Republic of China, PRC.

mainland China (noun)	Red China (noun)
a communist nation that covers a vast territory in eastern Asia; the most populous country in the world	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world