When it comes to picking a Fabric D for the Penny Quilt - I admit to being a little basic. My go to colors are white, off white, or light gray, but you can be as creative as you dare when it comes to picking sashing fabric!
The two qualifications that I would look for when picking a fabric: it should stand out from your other fabrics and it should coordinate (look good) with each of your fabrics.
Let me show you some examples of a quilt mocked up with different sashing options. These mockups use Christmas Morning by Lella Boutique and use a range of lighter to darker solid fabrics for Fabric D.
There really isn't a right or wrong choice here - we might prefer a different mockup of the quilt and that is ok. When I made these I expected to like the light gray best, but ended up preferring the mossy green color.
Which one do you prefer?