When the hair fibers are made from keratin or even a cotton synthetic, hair fibers easily blend in and fill in thinning areas. Hair fibers may be noticeable if you try to cover larger bald areas. In addition, fibers have to be matched to your hair color. They typically appear darker and don’t hold the light as real hair fibers, however. This is why they should be applied as close to the scalp as possible to avoid dulling your natural hair strands.