Bob's Prismatic Ward (all damage dealt to enchanted creature by sources of the color of your choice is reduced to 0) is moved from one creature to another. Any permanent changes to the enchantment (such as a color change) are erased, and Bob chooses which color of damage will now be reduced to 0. (He can keep it the same if he wants to.) However, his Ivory Cup (": Gain 1 life for a successfully cast white spell") can't be used, any enchantments on his Prismatic Ward remain in place, and so on. Also, he can move the Ward onto a Homarid Warrior, even if the Warrior's ability has been played and it can't be the target of spells or effects.