When you replaceitem a shield into the head of any entity that can have the shield in its head slot, it changes colors:
(Tested on Skeletons and Zombies, but should work on others)
/replaceitem entity @e[type=Skeleton,r=5] slot.armor.head minecraft:shield
/replaceitem entity @e[type=Zombie,r=5] slot.armor.head minecraft:shield
Video/gif:
1 (Zombie Only): http://gfycat.com/LeadingSoupyAkitainu
2 (Zombie and Skeleton): http://gfycat.com/MiserableFarEwe
Confirmed. But when I test it, it seems that it changes only when it is daytime and not other causes. Only things that can damage the zombie by head, like sunlight.