Arellior's Shield of Retroactive Resistance

You can target an ally within 30ft of you. The target gains resistance to the last damage type it took damage from within the last 5 rounds (30 seconds), and advantage on saving throws to avoid that damage type for 1 minute. If the target's most recently received damage type was dealt at the same time as another damage type (multiple damage types in one attack), the target gets to choose which of the damage types they gain this spell's effect in. If your target already has resistance in the damage type this spell provides, they gain immunity for the next time they take that damage type within the one-minute duration of the spell.

The target is sheathed in energy corresponding to the damage type they gain resistance to:

Bludgeoning/Piercing/Slashing: Grey

Fire: Red/Orange

Cold: Cyan

Lightning: Blue

Thunder: Purple

Poison: Dark Green

Force: Scarlet

Radiant: Yellow

Necrotic: Black

Psychic: Pink

Acid: Light Green