I think any color that is in the lovely design would make a nice border. I like how you did the opening on the top. A little bit of color there defines the opening nicely.
I think you are looking for a color for the bottom border. I personally think I would stay with the purple but of course, it is your choice. That's why we make things. We can do as we please, so do what makes you happy. Your cover is very nice. Did you have a pattern, or make your own?
I would not want white where your had grabs the kleenex, I like the color there. But for a border below the flowers - yes, I think white to keep the flowers popping rather than the border.
It is a lovely box.
Well done. Makes one look forward to spring. I also like the border the way it is.
Nice work! I like the border as is although my first thought would have been white,