Is there a plugin that allows group members to change there username color?
No quite that, so they can change the color to red instead of showing blue.
This plugin can do like you are asking.
he wants it so the user can change their own username colour no per group but per user
If i understand correctly
also that plugin dosen't work right on 1.6
At the moment, unfortunately (unless there is a plugin), you gave to create a group for them