The shirt that I'm wearing is mine

(My shirt is red.)

Yours is blue.

(Your shirt is blue)

He's not wearing his.

(He's not wearing his shirt.)

Hers is blue.

(Her shirt is blue.)

The strawberry is red.

Its skin is red.

("Its" is hard to use as a possessive pronoun. It's often better to use the possessive adjective instead.)

This is our website.

This website is ours.

Those uniforms of yours look great.

My dog is brown.

Theirs are gold.

Note: it's also possible to say, "Theirs is gold," if they have just one dog. A possessive pronoun is singular or plural depending on the antecent. An "antecedent" is the word or words to which the pronoun refers.