Once Urza is in play, your mana explodes, letting you chain spells and hold up interaction effortlessly
You may put a creature card with mana value 4 or less from among them onto the battlefield
If a permanent has no stun counters on it, it will untap as normal
While it might be unlikely for Muldrotha to see play, I could imagine it as a 1- or 2-of in a BG Delirium-esque midrange shell, perhaps after our current (The Scarab) God of Standard rotates this Fall
And if you have mana acceleration by something like a Sakura-Tribe Elder, then you can return the STE to your hand on turn four and recur something else the next turn and then cast the STE and use it again
You can't choose "multicolor," "gold," or "colorless." If there is somehow no chosen color, Heraldic Banner's static ability has no effect, and its mana ability produces no mana if you activate it