Command Tower

{T}: Add one mana of any color in your commander's color identity.

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 the battlefield with an additional +1/+1 counter on it.

Arcane Signet

Rogue's Passage

{T}: Add {C}.
{4}, {T}: Target creature can't be blocked this turn.

Tome of the Guildpact

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

Seal of the Guildpact

As Seal of the Guildpact enters the battlefield, choose two colors.
Each spell you cast costs {1} less to cast for each of the chosen colors it is.

Chromatic Lantern

Lands you control have "{T}: Add one mana of any color."
{T}: Add one mana of any color.