Red China vs mainland China

Red China and mainland China both are nouns.

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

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

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

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

Red China (noun)	mainland 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