The preferred horse hair of professional violinists is said to be "White Siberian" horse hair. Adapted to the cold and harsh environment the horse's tail hairs grow sturdy and have the right grip. More on grip later. Next is other natural white horse hair from other sources, followed by bleached white horse hair and only a few use natural color hair. A couple of reasons violinists prefer white horse hair: Its flashy! The audience can see it in a dark concert hall. When you rosin up the white powder on the horse hair doesn't show. Cello and Viol players on the other hand tend to use Black horse hair. It is thicker and has more grip. Now about grip. If you hair a bow up but put no rosin on it and run it across a brand new string you would get no sound what so ever! The tiny ridging on a horse hair does a great job of holding on to the rosin but only in one direction so the horse hair has to be blended back and forth so those ridges can hold onto the rosin, the more rosin the more "grip" traction is supplied to the string. Modern science has tried to come up with a synthetic horse hair that tries to act the same but is also much sturdier with one of the most successful called Hervex. Personally I prefer Black but I've seen all of the above work perfectly well in different circumstances.