Try a fabric paint and paint over the lines. Those are typically heat set. It works and doesn't wash out. Ask me how I know, it was one of those LEARNING OPPORTUNITIES. I didn't do that, but I stitched w/a white thread on top and a dark thread on the bottom. Where I started and stopped w/the midarm, the bottom thread showed. That was the only way that I could figure to fix it. I actually had to mix 2 whites/off whites to get the color that I needed.