The red window will open when you obtain the seven effects that can be reached through the other two windows (Rabbit, Pudding, Mermaid, Angler Fish, Sickle, Hydrangeas and Surgical Mask). This page will offer information about how to obtain them.
Effects
There are a total of nine effects that can be found in Madoru-Mu
Rabbit
It can be attained by interacting with a white rabbit in the clover section of the Giant Garden.
Pudding
Interacting with a pudding on a plate in Dessert World will give you this effect.
Mermaid
Finding the five stands next to each other in the festive world and interacting with the one with the fish tail will give you the effect.
Angler Fish
Can be found by interacting with a small angler fish on the Ocean Floor.
Sickle
Can be found in the grave-stone world by interacting with a bloodied scythe
Hydrangeas
Found in Lilypad World, interact with the rain plant there to get it.
Surgical Mask
You can get the surgical mask by interacting with the lone nurse in the hospital area.
Rokurokubi (Floating Head)
Found in the endgame. Interact with the headless person in the red door, then enter the green Uboa's 'mouth'. Walk up the red carpet to the headless princess and you will receive the effect.
Steering Wheel
Red window → Black door → Signals World → Stop Sign → Car.