In the first “Insidious,” a young Dalton draws a red door with crayons, then we see the actual thing when Josh enters The Further. “The red door” looks different in “Insidious: Chapter 3,” as Elise enters The Further through red elevator-like doors. “Insidious: The Last Key” — a prequel — expands the symbolism even more, imagining a Further hallway full of red doors, each giving a window into the real world. These red doors, we learn, are controlled by the Keyface Demon. Elise inadvertently leaves the red door to the Lamberts’ attic open, suggesting that she’s accidentally responsible for kicking off the events of the first film.
The red door in “Insidious: The Red Door” seems to function differently than in the others. This time, Dalton doesn’t have to literally open a door to let something through; simply by accessing the suppressed memory — by drawing it — he activates it. This is clear because the bathroom ghost orders him to close the door before we’ve actually seen a physical door being opened, suggesting that Dalton’s art is itself the portal to The Further.
As a result, Dalton painting over the artwork in the film’s climax has the effect of damaging the door in The Further. However, Dalton tells Chris that he doesn’t want to destroy the memory. Instead, the blacked-out door becomes the background for a new painting, one memorializing his father’s bravery. The “door” is still there, but this happy memory prevents it from being opened.