If you have two commanders, the last 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, Opal Palace's last ability produces no mana. It doesn't produce {C}.

If you don't have a commander, Opal Palace's last ability produces no mana.

The "number of times it's been cast from the command zone" includes the most recent time. For example, the first time you cast your commander from the command zone in a game, if you spent mana from Opal Palace's last ability to do so, it will enter the battlefield with a +1/+1 counter.

If Opal Palace's last ability produces two mana (most likely due to Mana Reflection) and you spend them to cast a commander, that commander enters with two counters for each time it's been cast from the command zone this game.