Arcane Signet

Artifact

: Add one mana of any color in your commander's color identity.

If you have two commanders, the ability adds one mana of any color in their combined color identities.
If your commander is a card that has no colors in its color identity, Arcane Signet's ability produces no mana. It doesn't produce {C}.
If you don't have a commander, Arcane Signet's ability produces no mana.