Faith’s Shield targets a permanent regardless of how much life you have. If that permanent is an illegal target when Faith’s Shield tries to resolve, Faith’s Shield won’t resolve and none of its effects will happen. Neither you nor any of your permanents will gain protection from a color.

A player with protection from a color can’t be damaged, enchanted (including by Curses), or targeted by anything of that color.

You choose only one color (not one color per player and permanent). You choose the color as Faith’s Shield resolves.