Firstly, they don't use lime green. I's just the lightning in the pic. It's more a light kelly green. Secondly, their red kits are just a traditional thing. They started using them from early on and just never changed to their official club colours of green, black and white. It's the same with Hamburg, where just the socks are in their club colours blue, black, white.
I guess that's kind of a German thing, where clubs wear different home colours than their official club colours. In my personal opinion I really like it for Hannover because their stadium looks asthetically nice with all the red shirted fans with green, black, white flags.