You have to be dressed like a leprechaun (with a hat, cape, and stick). Then, you have to save the second girl leprechaun (I cannot remember what number she is). Her daily task is to collect pots of gold at the end of rainbows. If you hit one of the pots, which show up every once in a while in the fields (costs 5 energy to whack,) then you complete the “My Rainbow!” quest. Hope this helps!