The nut slots and bridge saddle slots are cut for their regular set (45-128 if memory serves on a five-string) so the string diameters are pretty close. I ran 50-135 roundwound sets more than once and I could sense no binding or grabbing. Your results may vary. I'd pencil-lead or use your lubricant of choice in the slots if you're so inclined. Most makers 'eyeball' the slots (cut by hand), so generally you've got some wiggle room regarding string sizes. Try it, I'd think you'd be fine if your experience is anything like mine.
Bear in mind that a flatwound set will generate more tension than roundwounds, so you may need to fine-tune your truss rod / neck relief if sensitive to small changes.
I'd vote blacks. Who was it in the old days made those awful-looking maroon-wrapped tapewounds? I still remember being horrified at how hokey they looked !
Joey