This shield spell is quite powerful, and popular in Wizard duels. When cast, it creates a semi-translucent “sheet” around the caster or the caster's target, which flows and billows like a sail, always attached to its target. Every combat round, it changes its color and proceeds to block incoming damage. However, the shield only blocks a single type of damage each round- it shifts to block the first type of damage and doesn't change again until next round. This spell only blocks 5 spells or attacks before dissipating.
All sources of the same damage type are protected against by the power of the shield. When blocking physical attacks, the shield is gray in color and grants +4 AC for the first and every attack put against the character. When the sheet is blocking elemental or magical damage, it turns an appropriate color and instead reduces incoming damage by -4. If it's absorbing fire damage, turns red and reduces damage from a dragon's breath along with any other sources of fire that round by -4, etc. When dealing with esoteric sources of damage, such as from otherworldly creatures, mutant powers, or divine smiting by the Gods, the shield flickers between several crazy color and texture patterns and blocks only -2 damage or none at all, depending on rarity and strangeness.
The shield here also seems to have a bit of an animal mind attached to it. By making a saving throw versus spells, whoever is holding the shield can command it to leave them and go to someone or something else, protecting them or it from harm. On a failed saving throw, the shield cannot leave that character and becomes stuck to them until it ends.