dark green on turtles shell?

by naomi
(kent (uk))

hi, i have just taken on 2 male map turtles from my friend who did not look after them proply. one had really bad soft shell and the other was going the same way. i went out and brought heaters lighting and a ubv light as well as the right food etc, and im pleased to say the shells are hardening they are eating fine and are quite greedy and they dart around the tank really quick....but one of their shells has started going a dark dark green around the edge's just wondered if someone could help me on what it is?

many thanks

naomi :o)