10 Nov 2020
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}.
10 Nov 2020
If you have two commanders, the ability adds one mana of any color in their combined color identities.
10 Nov 2020
If you don't have a commander, Arcane Signet's ability produces no mana.