My mum’s bag is blue.
Is is an auxiliary verb in that sentence? If not, what is it?
Is “is” an auxiliary verb in the sentence “John is working now”?
was suggested as a possible duplicate, but that post doesn’t answer this question because it is about a different kind of sentence (“John is working now” is a progressive/continuous construction, but “My Mum’s bag is blue” is not).
“Is” is a verb of being, also called a “copula”.
noun Logic & Grammar
a connecting word, in particular a form of the verb be connecting a subject and complement.