Check out a color wheel online! That always helps me with colors. (or my husband who is an artist) Or you can go into a Paint program and play with colors on a page and see what you like! 
I think that a soft green would compliment well, even a purple. But, then you asked for contrasting?? Orange! I don't think you want that!
I think either a pastel type of green or blue.
I'm actually making a cake this weekend with burgundy roses and mauve roses.
My aunt's wedding cake was iced in champagne colored bc w/ two shades of mauve roses around the bottom borders and periwinkle piped embroidery on the sides and tops. You wouldn't think those colors would work together - but they did!
Personally I think a light sage green looks great with mauve. Also, mauve and burgundy look great together.
I often find great color ideas on the Sherwin Williams webpage. It's a paint website. Go to this page, and click the "Launch Color Visualizer" button. Pick a color, click it, and it throws up color combinations to compliment every possible shade.
http://www.sherwin-williams.com/do_it_yourself/
I think I'd go with something green-ish or blue-ish...maybe aqua?
I guess it depends on what you're wanting to accomplish.
The green or blue would provide contrast whereas if you used a burgundy or peach with the mauve, it would still compliment it without having the striking contrast.
Thanks for so many great ideas! I decided to use a sage green. I put fondant whimsy roses w/ leaves on a square small stacked cake, with cut out mauve & sage green hearts on the side. It is in my pictures. I ran out of time and my border looks messy, but otherwise its a cute cake. 
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