Text (SA+errata): 1/2. ; Artifact spells you cast cost {1} less to cast. [Oracle 2009/07/08]
This reduces the amount of mana you have to pay. It does not actually alter the mana cost or converted mana cost of the spell. [Shards of Alara FAQ 2008/09/23]
This will not reduce any colored mana cost to play spells. [Shards of Alara FAQ 2008/09/23]