The Si locus determines whether a rabbit has silvering, which causes white tips of hairs.
No Silvering (SiSi)
Most colors lack the Silvering gene. These are normal colored rabbits. The Silvering gene may be excluded from genotypes, which might assume that the rabbit doesn't (or does) have Silvering depending on the breed.
Non Silvered Colors
Rabbits with the Silvering gene will have white-tipped hairs in the fur. Unlike Steel, which can be Silver-tipped or Gold-tipped depending on the C locus gene being expressed, Silvering is always White. Silvering is also a separate gene from Steel and can combine with any other color, even Otters, Tans, Torts, etc. (although these colors are rare to unheard of).
The amount of Silvering can depend on modifiers. Breeds such as Silver and Silver Fox have lighter silvering, whereas breeds like Champagne D'Argent, Creme D'Argent, and Argent Brun have much more silvering, to the point where rabbits of those colors may look grayish white. The Champagne color can also be bred into other breeds, such as Netherland Dwarfs.
Silvering tends to be less prominent around the ears, nose, and eyes. It also may not be visible on kits, until the rabbit is older and developes the silvered hairs (this can cause a drastic difference in appearance for baby Champagnes and other heavily silvered rabbits who are genetically Black underneathe all that silvering!).
The US recognizes Champagne D'Argent, Creme D'Argent, and Argent Brun as three separate breeds. However, these breeds are very similar. They are all heavily silvered and are three separate ciolors underneathe the silvering (genetically). Champagne D'Argents are silvered Blacks, Creme D'Argents are silvered Oranges (non extension agouti), and Argent Bruns are silvered Chocolate. There are other Silvered colors that are more common outside the US. When heavily Silvered, they tend to go by unique names. Bleu refers to a silvered Blue and Saint Hubert is a silvered Chestnut, for example.
The Silver (a more lightly silvered breed) also has a unique name for its Chestnut, called Brown, which is genetically a Silvered Chestnut Agouti (same as Saint Hubert, the more heavily silvered version).