I second what Dunster has said. Using a smaller scale print and limiting yourself to a monochromatic color scheme will calm a quilt down. Also less contrast will help. Notice how Dunster's last quilt looks calmer than the first two. It is because the blues she used are much closer together. You could also use different colors, but similar values of the colors to achieve the same effect - like using pastel yellow, baby blue, and mint green together with a cream background. If you haven't purchased any fabric yet, try laying bolts of fabric together in the store and see how well they achieve your desired look. You might also enlist help from the person at the shop.
Darren