I have had problems getting pencil marks out of quilts which really upset me. I had the problem with the gray fons and porter (on a light fabric) and then with the yellow sewline. I tried everything you could think of. The thing that worked best was from Quilter's Rule and was "marking pencil removal". I can't say that it got it entirely out, but it removed it better than all the other products I tried. After the fact, I found that the problem is when you actually stitch on the lines the pencil lead gets into the thread and that's what makes it hard to remove. I have learned to never use the yellow for sure. The other colors seem to erase with a fabric eraser.
Hope this helps...