Thank you so much for the feedback, everyone! I'm so relieved that I'm not the only person who has had this problem. In response to some of the replies here...
1. The Steel fabric on my back is
not all from the same bolt. I made purchases from two separate locations, about 2 months apart.
2. I flipped the problem block over (without changing the orientation), and the colors in my top row appear to be the same now. I had some trouble resewing that one block, so I ended up redoing the entire row. Even though I marked the wrong side, I couldn't figure out how to position the fabric so that the seam ended up on the correct side.
3. When working with solid fabrics, how can I be sure that I'm consistently cutting along the right grain line?
Thank you, thank you, thank you for the sweet comments about the back! I'm happy with how it turned out, and I can't wait to start quilting! Here's a peak at the front:
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