Color and colour are two spellings of the same noun (which can also be used as a verb). The spelling depends on the type of English.
In American English, you use “color.”
In British English, you use “colour.”
Australian English mostly follows UK guidelines, so colour is standard.
The same difference applies to similar words, such as “behaviour or behavior,” “honor or honour,” “labor or labour,” “favorite or favourite,” “favor or favour,” and “humor or humour.”
It’s important to choose one type of English and use it consistently. The QuillBot Grammar Checker can help you with this.