How to Farm Rainbow Coins
To farm the rainbow coins, prepare a lots of normal coins, and the put them into the dice repeatedly.
The dice will process each coin seperatedly. Since there are slightly more chance for the dice to return double coins instead of return nothing, and there are a chance for it to yield an additional rainbow coin.
You can constantly get rainbow coins this way.
You can counter the dice cool-down by use more than one dice. For me two dices is enough.