It's true that PA is a no-nonsense person whose priorities are strictly work-driven, but I think a little change-up in her appearance wouldn't do any harm and hopefully not be too earth-shattering for the public. It would be a shame to be so locked in a look and style that now she couldn't change it even if she wanted to.
Do you remember in the 1980s i think it was, for some evening event, Princess Anne had her hair up (as usual) but it was coloured bright pink at the ends? So if she ever changes, it will be because she wants to.
I personally think she should leave the style as it is (it is so 'her'), but lighten the colour. THat would improve the overall look IMO.