Glass of the Guildpact

Multicolored creatures you control get +1/+1.

Civic Saber

Equipped creature gets +1/+0 for each of its colors.
Equip {1}

Brave the Elements

Choose a color. White creatures you control gain protection from the chosen color until end of turn.

Obsidian Obelisk

This artifact enters tapped.
{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to cast a multicolored spell.

Forest

Tome of the Guildpact

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

Seaside Citadel

This land enters tapped.
{T}: Add {G}, {W}, or {U}.

Plains

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.)

Bant Sureblade

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

Knight of New Alara

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

Brokers Charm

Choose one —
• Target creature you control gets +1/+0 until end of turn. It deals damage equal to its power to target creature or planeswalker an opponent controls.
• Destroy target enchantment.
• Draw two cards.

Esper Stormblade

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

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.

Island

Bant Charm

Choose one —
• Destroy target artifact.
• Put target creature on the bottom of its owner's library.
• Counter target instant spell.

Naya Hushblade

As long as you control another multicolored permanent, this creature gets +1/+1 and has shroud. (It can't be the target of spells or abilities.)