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What is a silver fox?
Genetically, a silver fox is a tan mouse (at/a C/- or at/at C/-) which has been diluted by the chinchilla gene (at/a cch/cch or at/at cch/cch). The chinchilla gene dilutes all pheomelanin (red/yellow pigment) in the coat to white, but leaves the eumelanin (black pigment) mostly untouched. They aren't simply a white bellied tan, however, the standard is different. For example, the silver fox standard cites white ticking over the flanks and rump as an added beauty, whereas on a tan ticking on these areas is a fault. It's this ticking that gives the variety the name 'silver' fox, as white ticking is known as 'silvering'. The mouse variety standard comes from the much older standard for the silver fox rabbit. (NB - this is known as silver marten in the USA, the American silver fox is another breed of rabbit entirely.)

What colours can they come in?
True silver fox can be bred only in black, chocolate, blue, and lilac, as these are the only varieties where the top colour is not diluted by the chinchilla gene. Adding the tan gene to any c-locus variety, such as Siamese or cream, will result in a white bellied mouse, but these aren't true silver foxes. Adding the chinchilla gene to an agouti based variety such as agouti or cinnamon will bleach the pheomelanin from the top colour as well. Agouti becomes silver agouti, agouti tan or white bellied agouti becomes chinchilla, argente becomes argente creme, and cinnamon becomes an unstandardised pale browny-white. Pink eyed self foxes are impossible as the chinchilla gene dilutes the top colour of those varieties too. Dove tan becomes a bright yellowy cream with a white belly, which is unstandardised but actually very pretty indeed. Red fox (Ay/at c[ch]/c[ch]) is a marten sable; the tan gene combined with the dominant red gene turns the demarcation line into shading, and the chinchilla gene bleaches the red pigment. At this time, with the genes that currently exist, I don't know of any way to make a red fox. Maybe a white bellied self gene will mutate one day; making red, argente, dove, and champagne foxes possible.

Tips for breeding silver fox:
Silver fox are one of the simpler varieties to breed. They breed true (assuming they have two tan genes, not just one), so there are plenty to choose from in each litter when selecting the next generation of breeding pairs or trios. Like any variety, attention needs to paid to the faults as these will take over a line if left unchecked, and matching faults should not be paired together. Foxes can excel in type, size, eyes, and ears, and this should not be allowed to fail in a line.

Common faults in silver foxes:
Most faults in silver fox are found on the under. A coloured spot in the white of the throat, a yellow or cream tinge to the white under or at the demarcation line, a line under, and thin belly fur which allows the dark undercoat to be seen through the white are all highly undesirable on a show silver fox. The chinchilla gene can have a very mild bleaching effect on the top colour if not kept in check and this can turn black silver foxes charcoal in colour; this means they will fail top colour on the show bench. A lack of ticking is not a fault as such, as the standard only says ticking is an added beauty, but poor ticking will probably mean that mouse won't compete well against others.

Above: one of my first blue foxes. The creamy yellow tint to the white can be seen at the demarcation line and tail base.
This has since been improved by selecting for whiter unders in each subsequent generation.

Outcrossing silver fox
Chinchilla would be the best non-fox outcross for silver fox, it would do no damage and might actually help the ticked effect of the silver fox. Failing that a black or chocolate tan with epic tan and faults such as tan ticking over the sides and rump is a good outcross for silver fox, but this will affect the whiteness of the belly for a few generations. Pink eyed white can be used sparingly; this outcross will improve size and type but will weaken the top colour and thin the belly fur for a few generations, so is best only used if a type and size injection is desperately needed. Other than those varieties, there are no outcrosses for fox that won't set the line back a couple of years.

Showing silver fox:
Generally, only bucks are shown. Does have thinner belly fur and nipples, which ruins the pure white of the under. Bucks have much thicker fur underneath, no nipples, and are also prone to stronger, more prevalent ticking on the flanks and rump. White ticking improves with age, so the best examples are usually at least four months old. A good silver fox buck should have a long show life if kept in top condition and not overshown. For people who show monthly or more, a team of bucks is needed so that they may alternate shows to reduce stress. Alternating mice for showing will enable the mice to stay in much better condition long term. This means that the silver fox exhibitor needs to keep quite a lot of bucks, which will all need their own cage.