You have a tough choice here. The challenge is on! How about a medium blue blend on the back and top. Good luck and please let us know how it turns out.
The tension can be balanced and that can take many tries. It might seem impossible so don't give up. Do some practicing on some scraps of lights and darks with both threads. You could also try monofilament in the bobbin. Superior has a great mono thread.
The top is fantastic. Anxious to see it quilted.
peace