Collectors, on the other hand, could be choosing to chase this card down due to their love of the song or, more generally, the music of Mulan
Its a good option to destroy problematic nonbasic lands though, especially in singleton formats
nintendo.com.au/switch-online
Notice that indestructible is not the same as effects that prevent all damage: With indestructible, the damage is still being dealt (and thus abilities that care about dealt damage will still trigger)
Only creatures and lands Cards that mill yourself "Whenever a creature enters the battlefield..." triggers Click here for a text version of this decklist Click here for my EDH Commander decks
Therefore, for the situation stated in your question, red, blue, and black would count as 3 colors, so the effect would activate four times