It has been this way for quite a while now; happened right before the pink mystery boxes were released. (There was some color glitching going on with the Dreams shirts also I remember. ) It was never an intentional change as far as I know. The original Princess Hat was designed to go perfectly with the purplish area in the original dress colors, and I really miss not being able to wear them together.
Edit: Very interesting; from checking the VMK wiki, the hat and dress may not have been a perfect match even back then. Or maybe it was just display differences between monitors in those screen captures? Question's photo above looks more like the dress color I remember.
However my own photo seems to show a better match:
View attachment 3451