With white shepherds good pigment is desired - occasionally you will get dogs with snow nose - and that is okay, but a fully black nose is more desired tha that - and usually it is even better when the white extends all the way up the nose.
However - Liver and Blue noses are a DQ with most white shepherds and white GSDs. (however...I have never actually seen a liver or blue nosed white O.o)
Pretty much all the standards like as much black pigment as you can get (the ears, eyes, nose, lips, pads and nails). This is my girl - and she is a pretty good example of excellent pigment.
Here is an example of a snow nose :
(this picture isnt great - but on the nose you can see the black pigment is somewhat faded.
here's another of faded pigment / snow nose :
With snow nose it become very black again in the summer, and pales out in the winter
With dogs with actual faded pigment that is not prefered over dogs with good pigment. With dogs who are missing black over the majority of their nose or their whole nose, that is a DQ...having a really pink nose is usually not well liked either.
Once I get back home (I am at Newark Liberty International right now) I have a picture of a dog with severe snow nose (in switzerland I am fairly certain as bad as she has it it is a breeding dq).
Hope that helps