I love using variegated threads. Are you asking for the quilting or the piecing? If piecing you can use a blue, or grey or whatever. for quilting a blue and white variegated thread looks really great depending on how you quilt it. I would use a blue on the back, close to the color of your fabric.
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My newest Grandson, Caleb Austin, was born May 29th. I am now Grandma to 4 precious babies. I am so blessed!!!!