The kind I use is black. The hole in the middle is smaller than the hole in air tubing. The window spline has a smaller diameter. So, being black, not shiny, and thinner, it's less noticeable.
As I mentioned earlier I use window spline as well. The reason I chose the spline is because I thought it would have UV protection and not breakdown. Maybe it's a nonissue as it doesn't stay in place all that long. I've had in place on some of my trees for over a year and when I remove the wire the spline looks like the day I put it there. Any thoughts?
I didn't make up the window screen spline idea... I watched Diasako Nomoto wire a JWP, and he used it when he needed to guy wire a couple branches.
I was surprised at how thin of a wire he was able to use. That had nothing to do with the use of spline, per se, but since the spline is slimmer (and the interior hole is smaller) I've been using thinner wire.
I use silicon feul tubing from my other hobby, R/C planes. You can get several feet of it for fairly cheap at most hobby shops, and it is avialable in different sizes so you can match it to whatever size wire you use.
I've used an old serpentine belt from a car engine. The mechanic looked at me funny when I asked him if he had any used ones lying around. It has grooves on one side that the wire can bite into to help keep it from slipping.