Perhaps these candids from circa late 70s, early 80s might shed a little light...
Seems she did have a genuine blonde phase before returning to her natural hair colour (brunette). Perhaps it was a fashion for her at the time, she even went auburn at one point. One thing observant Joyce will note is that throughout her modelling career her hair length remained long when wigless regardless of how it was styled.
It appears she seemed use wigs more prolificly the further into her career she got. A big blonde wig first appears on her crown in the 60s (I think). Then a short dirty blonde wig circa early 70s. Later she uses a variety of blonde wigs and even a black one late 70s/ early 80s. To me it says she was either trying to be versatileas a model without drastically restyling her natural long locks or reinventing herself through her career. Probably a bit of both. The ratty blonde wig she utilizes in the 90s was definitely more the latter. And perhaps to a degree to help seperate her real life identity from her model identity.
Here are some of the different wigs she's used:
BTW, the giant bronze condom stuffed with walnuts in the 5th picture is Arnold Schwarzenegger