I would use several neutrals in the cool family, like postmark suggested. Black, white and grays would add a touch of sophistication to the PP and DD. Do they have to be SU colors? Gina K makes a nice gray - Moonlit Fog. And it's a nice heavy CS. It would look good against both PP and DD. You can scroll down to see it here (I don't know what your monitor will show, but it's a mid-gray in person, kind of a pewter tone):
Heavy Base Weight Colors
Best of luck with this unusual challenge! Love to see what you come up with!