Multicolored spells cost less to cast.
"It's a tool for aiding my other tools," explained Urza. "Now they'll work in harmony."
Illustrated by Dave Dorman · Invasion · #318
There are no rulings for Urza's Filter.
Spells you cast cost less to cast.
Devoid (This card has no color.)
Colorless spells you cast cost less to cast.
Artifact spells you cast cost less to cast.
Flying
Historic spells you cast cost less to cast. (Artifacts, legendaries, and Sagas are historic.)
Creature spells you cast cost more to cast.
Nonartifact spells cost more to cast.