Learn Danish: Naboen maler porten blå. - The neighbor is painting the gate blue.

The adjective blå is an objective predicative because it describes the object of the sentence, porten. Since porten is a common gender noun (en port), the adjective stays in its base form blå. If the neighbor were painting a neuter object like hegnet (the fence), the adjective would have to agree with the neuter noun and become blåt.

Since porten means "the gate" and is definite, shouldn't the adjective get an -e ending?

No, and this is a common trap! When an adjective describes a noun across the verb as a predicative, it only matches the noun's gender and number. It completely ignores whether the noun is definite or indefinite. You only add the definite -e ending when the adjective acts as a descriptor directly before the noun, like in the phrase den blå port (the blue gate).

Why does blå come at the very end of the sentence?

In Danish, when an adjective describes the result of an action on an object, it almost always follows the object. Just like in English when you say "painting the gate blue", the structure here is the verb (maler) followed by the object (porten) and finally the result (blå).

Does porten mean a regular garden gate?

The word en port usually refers to a larger, heavier gate, like a driveway gate, a farm gate, or a large archway door. For a smaller, typical wooden garden gate, Danes normally use the word en låge.