When you find a "???" Hidden Loot the type is usually set in stone, but the amount can be manipulated. If you get coins you can "cancel" out and re-select the hidden loot several times until you "roll" the maximum possible number of Rainbow Coins. Same for potions. Occasionally you find one potion, but there is a possibility to get 2 or 3. Simply cancel a couple times and re-select the Hidden Loot to maximize your findings.