On St. Patrick's Day Global? find the shards, make the orb and use it, then find the room at the end of the rainbow.
Note: Once you use the orb, you only have a few seconds to find the end of the rainbow.
Unsure if this is an alternate way to get the mark but when i got it i found the rainbow around the beltane continent. Deathangel n Gunbladee got it right after each other but ohmygod wasnt able to. might be lvl restricted or a short timer on it, you might also want to check the rainbow before getting the rainbow key as someone might have already looted the chest you dig for