I try to match the background color so it will more or less blend in with a small, tight zig zag stitch. Seems to do that nicely. I catch the applique with one stitch and let the other one go a thread outside it. It's already ironed to the back block with the iron-on fabric, but these will be washed often so want them to stay on for years.
Sometimes I want a black outline on strange, "artistic" and odd multi straight shapes and sharp color contrasts. Okay, I so I like odd things, doesn't take much to amuse a childish mind. And it uses up all sorts of scraps!! Think Picasso draws a bunch of quilting templates.