No two sets of eyes (which includes retinas plus all the processing that converts the input into internal scene representation) are the same.

This is especially frustrating with photography. No two people see a single scene the same way; the best you can do is take raw data and interpret it into the tiny, tiny display space the way you think best describes the scene. Yet do many think that the blandest possible JPEG render is the reference truth of what reality is actually like.

I wonder how important relatively speaking the individual differences (outside the extremes of color-blindness) are, compared to the fact that the customers are simply not sitting in such 'white rooms'.

well the whole point of trying to produce color corrected monitors was to avoid the whole process of printing things out and looking at them in white rooms as desktop publishing started to become a thing - high end publishers were (are?) very sensitive to their color fidelity, compelling color is very much part of what draws in their readers