You can couch any type of fiber: wool, cotton, rayon, synthetic. Single or multiple strands. Whatever suits your fancy. If you just want color, perle cotton is a good choice. I you are using a machine, you can use a simple zigzag stitch or any decorative stitch that will secure the fiber. the thread can match or be an additional accent. A couching foot is a huge help, but not necessary. The problem is when adding it to a quilt, you need to be aware of its washability. Your best bet is to wash the embellishment first. I would practice on a sample to figure out what stitch you are going to use and if you need a single or multiple strand of fiber. If you are applying it to an unquilted top, you will need to use stabilizer.