primary colour vs primary color

primary colour and primary color both are nouns.

Word	Noun	Adjective	Verb	Adverb
primary colour	Yes	No	No	No
primary color	Yes	No	No	No

As nouns, primary colour and primary color are synonyms defined as:

primary colour and primary color: any of three colors from which all others can be obtained by mixing

primary colour (noun)	primary color (noun)
any of three colors from which all others can be obtained by mixing	any of three colors from which all others can be obtained by mixing