1) Trade one of your most precious recurring dreams to the Gentleman of Thrice-Dark Woods. After he’s dined, and after you’ve realized what you’ve lost, he’ll gladly apply some sort of colored paste for you by way of apology.
2) Open a portal in the soft side of this world, your fingers tugging at the dimension’s fatty layers until you’re viewing the colorless obscenities that live outside all-we-know. (I assume you want WHITE hair?)
3) Close your eyes. There. Your hair’s black. Everything is black. Now press against your eyes. Your hair is now full of stars.