But the reason that they have this common model is because there is a red, chair-shaped object in the room. The common model isn't due to something internal to them, it's due to their all looking at the same external object.
What makes it red? The human eyes and visual brain center. What makes it a chair? The shape of the human body. What makes it one? The human perception and the human interpretation of it as a chair.
What makes it red is the wavelengths of light which bounce off of it. Our eyes and brain centre just make us aware of that external fact. Our perceptions and interpretations make it a chair, but that's only because we apply them to the chair shaped object which is sitting out there in the external world being completely unrelated to us.
The reason we have a common model of the chair is because we're all looking at the same chair. The rationale for that model is external to us, not internal to us.