Hi Penpen
Sorry for answering only now, but I was off for some time! 
Just a little input, appart from Blaster's comment.
The neger can be painted in two schemes:
a dark grey or a greenish/blueish grey.
It depended on the actual base from where the K-Verband was operating, beeing the most usual the dark grey version. 
Pictures from "Midget Submarines" and "Underwater Warriors", both by Paul Kemp, show them in a dark and bad weathered / dirty outfit.
I also have built the Ironside Neger and you can check it on the Gallery if you want...

You're right, it's not the best Neger on the market, beeing the Verlinden's one much better, including the torpedo attached.
JF-Models of Germany also make the Neger follow ups, also in 1/35: the Marder (beeing able to dive) and the super sized Hai (with extra bateries, making it a longer Marder)
If you have any other question, please, ask!!
Best regards from Lisbon, Portugal 
Skipper