Answer:
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.
What do you have when you're sitting down that you don't have when you're standing up?
Dies half its life. Lives the rest. Dances without music. Breathes without breath.
It is a path of cow juice spread across the sky but cannot be seen by the naked eye
You use it between your head and your toes, the more it works the thinner it grows.