The rule is not absolute. It is a rule of thumb.
In this case it depends on what characteristic the speaker is emphasizing.
She has fair, wavy hair sounds good. She has dark, shoulder-length hair also sounds good.
It means that when recalling her image in order to make a description, the speaker first thought about hair color, then her second defining chrteria was wavy or length.
If she thought of wavy first, then she would say "she has wavy, fair hair".
If she thought of the length first then "she has shoulder-length, dark hair."
I usually think of the person �s hair color first, then add other modifying adjectives.