I like this idea.

If you don't have enough of any one color I would use multiple colors cut in lengths to match the length of the outside blocks. I would use a color from each outside block to be beside that block. I would add in sections of the sashing fabric between the colors. I think that using the fabric used for the shadows would defeat the effect of the shadows.

Thanks everyone! I've decided to go with Madquilter's idea of making the whole quilt like a block with its own shadow! Might make the binding a little wider but I do have lots of that lovely gray fabric. Will post result sometime soon!

I like the dark gray. But depending on the backing fabric, that might be an option too.

Thanks for your response! :o This will be a wall hanging so backing color will not matter and will be muslin as is the sashing and many block backgrounds.

I was going to suggest same as madquilter. It would take some figuring to get it lined up but would look nice

I would make a two color flange binding, the flange in the shadow color and the outer / back part the neutral (white off white background color.)

Here is one of many tutorials on line https://www.youtube.com/watch?v=HAQWwwAa2K0

I vote for shadow color

I like the shadow color - nice. The blue grey color as suggested by Jennifer would be nice too!