Cant use a black border, it would be kill the black diamonds at the top and left side,
Cant use red, it would kill the red squares along the bottom and right side,
White might be ok for an inner border, but then what? maybe an outer border of the pieced squares? That would be pretty busy...
Im not sure it even needs a border.
I think that I would just bind it with the black polka dots.
Anything else would detract from the wonderful geometric movement of this quilt.