Some TMNT stuff really isn't for little kids.
Shades of Green is the title of a three-chapter story from the comic series Usagi Yojimbo by Stan Sakai. It is most notable for the guest appearance of the Turtles (all of them, this time) in the world of the story's main protagonist, Miyamoto Usagi.
While wandering about, Usagi and his friend Gennosuke inadvertently stumble into an area currently kept under the tight control of the Neko Ninja, who attack the two travelers immediately. Outnumbered, Usagi and Gen jump from a cliff causeway into the river below and are washed up near a village, where they are found by the village's wise man (rat), a mystic named Kakera, who declares that their coming here was ordained by fate: Their task is to protect the village from the predations of the Neko Ninja, who seek to capture Kakera and use his powers to restrengthen the Neko Clan.
When other samurai and mercenaries lured towards the village end up killed by the ninja, Kakera decides to summon ninja for help. Using four baby turtles, he conducts a mystic ceremony which brings the Ninja Turtles from their world into this one. First confusions are quickly settled when Leonardo acquaints his brothers with Usagi, whom he has met before, and upon learning about the situation, the Turtles pledge their support to Usagi, Gen and the beleaguered villagers.
Soon afterwards, the Neko Ninja commence their attack and, despite Usagi, the Turtles and Gen's stout resistance, manage to capture Kakera, critically wounding Leo in the process. Despite his injuries, Leo leads the others to the temporary base of the Neko Ninja (having followed one of their scouts just before the battle in the village), where they are set upon by the ninja. During the fight, Usagi manages to slip away and track a group of the Neko Ninja attempting to spirit Kakera away. He engages the escort, thereby inadvertently saving the life of the Neko Clan's leader, Chizu, who is caught in a power struggle against her underling Gunji, who desires the leadership for himself. Chizu and Usagi vanquish their opponents, and Chizu, whose late brother Shingen had fought once side by side with Usagi, leaves with a kiss on Usagi's lips, surrendering Kakera to his custody. After his recovery, Kakera sends the Turtles back home after they have exchanged invitations with Usagi and Gennosuke to meet again.