As stated in part 2 of the analysis, the second of director David Lynch's ten clues to unlocking the movie is, notice appearances of the red lampshade. The red lampshade, which is located somewhere where Diane is staying or living, is shown twice in the movie.
Top left: Mr Roque initiates the call chain by telling the person on the other end of the phone, "The girl is still missing." Top right: A man in a sleazy-looking place gets Roque's message via another man, then he hangs up and calls another number. The manner in which he dials the number, i.e., just 2 or 3 digits are dialed, indicates that he's in the same building or complex as the person whom he's calling. Above left: The phone by the red lampshade rings. However, we don't know who is being called until later in the movie. Later in the movie: Above right: We hear the phone ringing, and see the red lampshade again; then, Diane answers the phone. What the audience is supposed to realize is that it's the man with the yellow phone who's calling Diane, to tell her to go to the audition for the movie Adam is staging. Earlier (chronologically), Camilla called Diane on this same phone, to tell her that a car was waiting to take her to 6980 Mulholland Drive, where a party was being held.
Urban areas where prostitutes walk the streets are known as red-light districts. Therefore, the red symbolism indicates that Diane is, or formerly was, a prostitute or call-girl.
