According to the power Ranger books, to break a tie between Rita and Zordon, the pair had a series of coin flips with the six power coins. With Zordon becoming the winner, he kept the five (Red, Blue, Yellow, Black, and Pink) coins, and banished Rita to a space dumpster with Goldar and the others. However, she kept the Green coin for the one time she won.
Actually, the coin toss is only mentioned in an old children's book series. What really happened is Rita lured Zordon to a false peace treaty and trapped him in his timewarp. Zordon used his remaining strength to trap Rita and her goons in the dumpster. As for the green coin,I think Rita created it herself.
Ninjor says he created all the power coins, but it's debateable wether it was the green or white ranger coin he was talking about.