First of all...what gorgeous colors you've picked for this quilt. I just love all the fabrics! My first full size quilt was a YBR which I did in much more subdued colors...pinks, greens and browns for my Mom's 81st birthday. I, also, had issues deciding on the border(s). I ended up doing two narrow borders of brown and pink and the largest outer border in the green. The brown really helped frame the quilt since I had less brown in the blocks. I think your idea of the purple makes sense, but use it as the first border to really frame your blocks and then pull a couple of the other colors out for the outer borders...maybe the lime green and orange.