As Sanctuary Blade becomes attached to a creature, choose a color.
Equipped creature gets +2/+0 and has protection from the last chosen color.
Equip {3}
Activating Sanctuary Blade’s equip ability targeting the creature it’s already attached to won’t let you choose a new color.
If Sanctuary Blade is somehow attached to a creature without a color having been chosen for it, it doesn’t grant a protection ability to the creature. The creature doesn’t gain protection from colorless.