I'm sure your grandson and his bride will love whatever you make for them. However, keep in mind that they aren't quilters, so they will see this as an element of their bedroom rather than a stand-alone work of art. They might not want contrast - it will make the quilt less interesting on its own, but might make it fit with their room better.
I have made three different quilts for my own bed. The one that I find least interesting as a quilt is the one I most like on the bed. It's all light colours, with large blocks, and I find it very serene. The others with more contrast and more interesting patterns are less relaxing, so as much as I love them, I use them less.
Your grandson and granddaughter-in-law might not feel that way, but I would talk to them and see what their (and especially her) preference is. If they want low contrast, that's okay; they are just looking at the quilt differently from how we as quilters do.