I also vote zip ties. I've tried the plastic rings and they work but are more of a pain to work with. The big advantage with the zip ties is that you can adjust the size when you first put them on. With the rings you have to buy the right size.
I use the 7" ties. I find them easier to see on my full-sized fowl since they are thicker plus I think they are easier to use with my arthritis. That's just personal preference. I don't necessarily cut them exactly flush but you do want to remove the tail. I've never had a problem with one getting snagged, even with the band reasonably loose. I've used a lot of different colors, including bright red, and have never had a pecking problem with them.
Since all you are wanting to do is identify your friend's birds from yours is get one color. Look at their legs before you decide which color. Yellow on a black leg would really stand out, not so much on a yellow leg. Avoid clear, those are hard to see on any color leg.
If the chicken is still growing (the legs may continue to thicken even after they are a year old) check every month or so to make sure the bands are still loose. Of the two different methods I tried I much prefer the zip ties.
