If you use the blue which is the top plane of your windows then it will appear as if your quilt was a window in a wall because you have continued the "plane" on beyond the quilt center. This will give a completely different 3D image than in you just add a color that coordinates with your other colors. Your choice should depend upon what effect you want the total quilt to achieve. You have a marvelous 3D effect now. Do you want it to continue?