The door is painted = there is paint on it. This paint could be 1 week old, 3 months old, 20 years old. (Synonym: the door has been painted) The door is being painted = I/you/she/he/John paints the door now... is painting it (now at the moment of speaking, yes)

"The door is being painted" means that the action is happening now. "The door is painted" is possibly incomplete and so more than one meaning is possible: The door is painted every year- using present simple to show a habit The door is painted blue- means that the door is blue and is blue because it has been painted that colour.

Yes, you are correct about both. In the first sentence, painted functions as an adjective. When combined with the present simple, it becomes a fact that the door is painted. In the second, you can see the present continuous structure, which means it is ongoing in the present.