Funny thing, I was considering stitching the black but I didn't. I only used light beige and dark grey, leaving the black fabric to take care of the rest. I think that the faint difference you see is residue from the waste canvas (the material that gave me a grid to stitch in). So great suggestion, you have exactly the right idea

I definitely plan to do more of this, but it is incredibly time consuming. Even without the black stitching it was still a 50 hour project. My other one, the skylight screen, is going to be many hundreds of hours.