Tome of the Guildpact

Whenever you cast a multicolored spell, draw a card.
{T}: Add one mana of any color.

Hero of Precinct One

Whenever you cast a multicolored spell, create a 1/1 white Human creature token.

Gloryscale Viashino

Whenever you cast a multicolored spell, this creature gets +3/+3 until end of turn.

Guildmages' Forum

{T}: Add {C}.
{1}, {T}: Add one mana of any color. If that mana is spent on a multicolored creature spell, that creature enters with an additional +1/+1 counter on it.

Jund Hackblade

As long as you control another multicolored permanent, this creature gets +1/+1 and has haste.

Niv-Mizzet, Guildpact

Flying, hexproof from multicolored
Whenever Niv-Mizzet deals combat damage to a player, it deals X damage to any target, target player draws X cards, and you gain X life, where X is the number of different color pairs among permanents you control that are exactly two colors.

Knight of New Alara

Each other multicolored creature you control gets +1/+1 for each of its colors.

Tropical Island

Maze Abomination

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)
Multicolored creatures you control have deathtouch.

Pillar of the Paruns

{T}: Add one mana of any color. Spend this mana only to cast a multicolored spell.

Glass of the Guildpact

Multicolored creatures you control get +1/+1.

Bant Sureblade

As long as you control another multicolored permanent, this creature gets +1/+1 and has first strike.

Maze Glider

Flying
Multicolored creatures you control have flying.

Demand

Search your library for a multicolored card, reveal it, put it into your hand, then shuffle.

Supply

Create X 1/1 green Saproling creature tokens.

Supply // Demand

Urza's Filter

Multicolored spells cost {2} less to cast.

Maze Rusher

Haste
Multicolored creatures you control have haste.

Widespread Thieving

Hideaway 5 (When this enchantment enters, look at the top five cards of your library, exile one face down, then put the rest on the bottom in a random order.)
Whenever you cast a multicolored spell, create a Treasure token. Then you may pay {W}{U}{B}{R}{G}. If you do, you may play the exiled card without paying its mana cost.

Dragon Arch

{2}, {T}: You may put a multicolored creature card from your hand onto the battlefield.

Scuttling Butler

At the beginning of combat on your turn, if you control two or more multicolored permanents, this creature gains double strike until end of turn.

Rienne, Angel of Rebirth

Flying
Other multicolored creatures you control get +1/+0.
Whenever another multicolored creature you control dies, return it to its owner's hand at the beginning of the next end step.

Maze Behemoth

Trample
Multicolored creatures you control have trample.

Rampaging Monument

Trample
This creature enters with three +1/+1 counters on it.
Whenever you cast a multicolored spell, put a +1/+1 counter on this creature.

Maze Sentinel

Vigilance
Multicolored creatures you control have vigilance.

General Ferrous Rokiric

Hexproof from monocolored
Whenever you cast a multicolored spell, create a 4/4 red and white Golem artifact creature token.

Fallaji Wayfarer

Fallaji Wayfarer is all colors. This ability doesn't affect its color identity. (It can be in any deck whose commander's color identity includes green.)
Multicolored spells you cast have convoke. (Your creatures can help cast those spells. Each creature you tap while casting a multicolored spell pays for {1} or one mana of a color that creature is.)

Guildpact Paragon

Whenever you cast a spell that's exactly two colors, look at the top six cards of your library. You may reveal a card that's exactly two colors from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Invasion of Ravnica

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, exile target nonland permanent an opponent controls that isn't exactly two colors.

Invasion of Ravnica // Guildpact Paragon

Grixis Grimblade

As long as you control another multicolored permanent, this creature gets +1/+1 and has deathtouch. (Any amount of damage it deals to a creature is enough to destroy that creature.)