Seal of the Guildpact
Artifact
As Seal of the Guildpact enters the battlefield, choose two colors.
Each spell you cast costs 
less to cast for each of the chosen colors it is.
"I'd rather see the guilds contend together with words in the halls of power, than in the streets with swords."—Jace Beleren
Illustrated by
Commander 2015#54RareEnglishLegality
Rulings
You must choose two different colors.
Seal of the Guildpact’s last ability doesn’t change the mana cost or converted mana cost of any spell. It changes only the total cost you pay.
Seal of the Guildpact’s last ability can’t reduce the amount of colored mana you pay for a spell. It reduces only the generic component of that cost.
If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.
The cost reduction can apply to alternative costs such as evoke costs.
If a spell you cast has 
in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if that spell’s mana cost is 
and you’ve chosen red and green, you could choose 5 as the value of X and pay 
to cast the spell.