Nyehehe. So it's a mana that's simultaneously white, blue, black, red and green. Cool. Normally this will just be City of Brass, but... um... hmm, I can't think of any cases when that'd be any different actually.

...I think you can't get any mana from this in Commander unless your general is five colours. Which is a difference, but not in a good direction :P

Does "add to your mana pool" currently become R or G when it goes into the mana pool (not when you spend it)? I assume it works like that: the same at the moment, but might do more if they alter the rule so mana in a mana pool can count as multiple colours (just like a creature can count as multiple colours at the same time, not be half-and-half).

Yeah, if a Charmed Pendant or Elemental Resonance hits a Selesnya Guildmage then you get your choice of , or . That's why Wooded Bastion is templated the way it is (rather than the more elegant ", : Add to your mana pool.", which would be functionally identical but might mislead some players.) This card's design I'm assuming comes with a rule change to allow mana to exist in multiple colours until you spend it.