Another survivor will ask you to hunt down the leprechaun called "Greedy". That's pretty easy thing to do - just watch out for which location on the global map the rainbow appears. And then “I came, I saw, I conquered”! The good news is that the leprechaun will not be able to completely escape from the zone, even if you give him enough time. The bad news is that he won't always leave a corpse. If you're lucky, you'll be able to pick up some caps from Greedy and use them in the slot machine at the gas station. Bad luck? Don't sweat it, you can come across caps almost anywhere else!

In the slot machine, you'll sometimes receive clover, which you need to save up in order to gain access to new rewards in the event tab in the calendar. And the timer until a new rainbow (and until another Greedy) can be reset in exchange for coins. All in the same place, next to the slot machine at the gas station.