Seal of the Guildpact — ruling 1

You must choose two different colors.

Seal of the Guildpact — ruling 2

Seal of the Guildpact's last ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.

Seal of the Guildpact — ruling 3

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.

Seal of the Guildpact — ruling 4

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.

Seal of the Guildpact — ruling 5

The cost reduction can apply to alternative costs such as evoke costs.

Seal of the Guildpact — ruling 6

If a spell you cast has {X} 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 {X}{R}{G} and you've chosen red and green, you could choose 5 as the value of X and pay {3}{R}{G} to cast the spell.

Seal of the Guildpact : règles, fonctionnement et interactions (MTG)

Seal of the Guildpact is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As this artifact enters, choose two colors. Each spell you cast costs less to cast for each of the chosen colors it is.

How it works : Seal of the Guildpact

Original English text — Wizards of the Coast / Scryfall

#1November 4, 2015· Wizards of the Coast
You must choose two different colors.
No other card is named in this ruling.
#2November 4, 2015· Wizards of the Coast
Seal of the Guildpact's last ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
No other card is named in this ruling.
#3November 4, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#4November 4, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#5November 4, 2015· Wizards of the Coast
The cost reduction can apply to alternative costs such as evoke costs.
No other card is named in this ruling.
#6November 4, 2015· Wizards of the Coast
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.
No other card is named in this ruling.