The yellow background could be lit by another window or a different light source, so one could argue we don’t have a good reference to tell. But the point is that the “picture of a thing” is not “the thing” itself, and there is always a possibility that they are different.

You missed the whole point. If I take a white dress and then shine a blue lamp on it, then take a photo.The pixels will be 100% blue, but would that mean the dress itself is blue?