Well, this is because a hex nut has a purpose and it is for you to hex your lost 10mm wrench.
I would have thought that little black designer dress was correct. Designer little black dress feels incorrect. The L and BL in Little and black and then the Ds in designer and dress also lend an alliterative flow to “little black designer dress” that is not there for “designer little black dress”
I think the dress example is not super great because “little black dress” is an expression or something. It is like an accessible and inexpensive but still nice and fashionable dress.
So I think a little black designer dress would be a designer dress which happened to be little and black. A designer little black dress would be a dress of the “little black dress,” variety, which also has the trait “designer” applied.
I would have thought that little black designer dress was correct. Designer little black dress feels incorrect. The L and BL in Little and black and then the Ds in designer and dress also lend an alliterative flow to “little black designer dress” that is not there for “designer little black dress”