The title made it sound like a movie about prostitution
And it's not like there's any need for this title. Spoilers: The red lights in the movie are purely symbolic.
shareI initially thought of "Red Lights" as in traffic stop lights at an intersection. I guess it was referring to the red light above the door where Silver was supposedly reading people. Also, many seances use red lights (like in the first seance scene).
So not purely symbolic.
Margaret says to Sally when she is taking photos of people on the street that she is looking for red lights, that is things that shouldn't be there.