What is your granddaughter's favorite colors? Go with that!![/quote]
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That was my idea too. This stage of her life is limited, she'll move on to other living spaces and this quilt will have to fit in with them all. I'd suggest black, white with a touch of red. These will more or less fit in everywhere she goes in her life.
And be sure to label it, since things seem to want to migrate in shared quarters, and come and go visitors.