This is a mirror...
From the time he was very young, he knew what a mirror was.
He was handed a mirror one day and told: "This is a Mirror."
On the surface, he saw his face. This is a mirror he thought. Ok
When he looks at a mirror, he sees his face; or from the side he sees the wall and a bit of the door; whatever. The surface of the mirror is never the same.
When he was five or six, he was told: "This is the colour green," and he knows what green is now. But all colours are reflections of light. So the Green that he knows may look entirely different to someone else. To her for instance.
He will never know, because the light that reflects will always give him a certain image of green. His friend may look at something green and see something that he would consider to be blue. But she knows it to be green and sees it that way.
There are no guarantees that green looks the same for any two people.
He can make a green light, which relates to the heat of the wiring, or tint of the cover, or even a chemical reaction. But it may look to her. It may look to her like blue does to him. Yet, this specific heat, this chemical reaction, or even the tint of a window or cover, will always look His-Blue to her, and still she will always call it green.