Purple, white and black sound like a good colour combo. I use a ton of different flower combos in my forest town Tinytree and I think it looks pretty good. In my 2nd town town I'm sticking to purple flowers and white pansies and I love how cohesive everything looks so far. If you get too bored of one colour combo, maybe have a secret designated area in town that is a whole other colour combo? Like an area of forest with just a log bench surrounded by jacob ladders and mushrooms <3 I'm planning on doing something like that in my 2nd town.. I'm thinking of making an area that's only dandelions and yellow flowers!