When I marked diagonal lines for HSTs, I just used a regular mechanical pencil. It didn't drag and made a nice, thin visible line.
Kassaundra, it's true that Bonnie's quilts don't require busy fabrics, because the patterns are complex in themselves. There were lots of quilts last year made from constant fabrics that read as solids, and they were beautiful. I think you will be happier with the yellow, because it will allow the pattern to emerge