I just fell in love with my flower pins all over again, so I'm learning about them, too.
I think that you would be fine with a black flower pin like this one. It's large (6" diameter), so it will make a statement. Also, the clip-on back has limited versatility on tops, but would be great for attaching it to other pieces. If you're not sure, start with a smaller flower pin, maybe a 3" diameter.
You could also do purple-on-purple, but that's trickier.
Yep black would work! Gold could too... or maybe a red, depending on the shade of purple. A lot of the flowers I've seen have been BIG - so I'm not really sure there IS a "too big" so go for what you like!
LC... now how did you know I was making flowers for Christmas? Here are a few other tutorials: