Adding fibers dramatically reduces the likelihood of the product cracking. Fibers are also far superior to wire mesh for products like stepping stones and most other mosaic work. Don't waste your time, materials, and reputation using wire. I like nylon far better than polypropylene since it is much more flexible and lays into the mortar better without leaving a hairy surface. If you're concerned about durability over centuries, glass or carbon fiber is probably the way to go.
Pigments come in a wide variety of colors and forms.
Some which would be too expensive for most other cement-based applications are within reason for small mosaic projects. Learn about what you're using before you use it and take appropriate precautions. Release Agents can have a dramatic effect on the quality of the final surface of your mortar. You can use readily available lubricants like Pam, soy lecithin or Vaseline. However, more appropriate release agents especially formulated for cement will yield a smoother finish with noticeably fewer of those annoying little holes.