Oracle Text

At the beginning of your upkeep, you may have Shyft become the color or colors of your choice. (This effect lasts indefinitely.)

Card Rulings

10/1/2008 Shyft’s ability won’t let you make it colorless. Colorless is not a color.
10/1/2008 The ability’s effect has no duration. Shyft will remain the chosen color or colors until the game ends, it leaves the battlefield, or some other effect (such as the next time its ability resolves) changes its colors.