I feel less is more with the borders. I'd add very narrow sashing maybe 1.5" of one of the darker colors then 3" or so of your black followed by one more round of a lighter color in slightly wider width possibly 2.5" then a final round of black in whatever width you need to cover your mattress. and would bind in black also. Nothing could be easier than that, but to my eye, anything more risks distracting from that jaw-dropping, spectacular center! maybe even just one sashing strip would do it. if you lay it out on your bed, you can see where the art ends in relation to the drop of your bed and that might determine whether you want one or two inner sashing strips.