Sometimes silver fox get a white spot on their head, that just molt away with their adult coat. Check for white toenails though, those are a disqualification, and aren't the best to use in breeding if you plan to show.
How soon they get their silvering doesn't matter. It's the evenness and amount of silvering that does. Some judges want more, some want less, so pick what you want. I'd try to select away from ones that have very dark heads, though, if you were breeding for color
The most important thing for showing is body type. You want ones with a meaty body, very wide, and deep over the shoulders and hindquarters. They're supposed to have the same body type as new zealands (here are some pictures of good show quality new zealands:
http://www.newzealandrabbitclub.net/Photo%20Gallery.htm).
For meat breeding, a fast growth rate (the biggest kit) combined with the ones with a good body type would make the best meat breeders.
In the end it's really just whatever you like. If you want friendly rabbits, pick friendly rabbits. If you want big rabbits, pick big rabbits. etc <br /><br /> -- June 29th, 2017, 1:33 am -- <br /><br /> Vienna is a dominant gene (maybe co-dominant, if you're talking about bew), and it doesn't run in purebred silver fox lines. If it does really end up to be vienna, then your silver fox aren't purebred.