My father painted the door yellow.
Yellow is always an adjective modifying the noun “door”. The placement of the adjective should not change its basic function. Yellow is never an adverb.
An adverb must answer one of these questions:
When did your father paint the house?
Where did your father paint the house?
How did your father paint the house? How refers to the manner, the speed, and not the color of the paint.
Why did your father paint the house?
Under what conditions did your father paint the house?
The adjective “yellow” would not be able to meet any one of these questions.