Mirth enjoys a little humor in his magic, as does Merlin. In Chapter 1 of Mage, Mirth uses his magic to levitate and play with a Rubik's cube while talking with Kevin (Wagner, 1:17). Similarly, Merlin likes to have a little fun with magic, as illustrated in The Once and Future King, by T. H. White:
"Testimonials," said Merlyn, holding out his hand.
Instantly, there were some heavy tablets in it, signed by Aristotle, a parchment signed by Hecate, and some typewritten duplicates signed by the Master of Trinity, who could not remember having met him. All these gave Merlin an excellent character.
"He had 'em up his sleeve," said Sir Hector wisely. "Can you do anything else?"
"Tree," said Merlyn. At once there was an enormous mulberry growing in the middle of the courtyard, with its luscious blue fruits ready to pattern down.
"I serve the weapon and its wielder. And so, you see, we have always been student and teacher. King and wizard. Hero and mage."
So Mirth may not be technically the reincarnation of Merlin, but, more, the continuation of the spirit that is both he and Merlin. As Mirth puts it in chapter four:
"We've all had other lives, Kevin. I just happen to remember all mine. Anyway, at one time I had a weakness for very beautiful women. *Chuckle* 'So it goes!'"
The color of Mirth's magic is green, because, as he puts it in the eighth chapter of Mage:
"Magic, itself, is best described as a river. Any user of its water must dip into its swift and restless depths. Its color is then perverted, depending on its use. Green is pure, the river is green. I am green. I am, after all, the World Mage. You see, the river flows through me freely. I don't have to dip into it. I'm its faucet onto this world."