A silver has a a silve -grey underfur, whereas most whites have a white underfur. You do not see many that have a uniform silver underfur.
White with dark guard hair is a white animal with dark tips. The longer guard hairs are grey to black and stick out over the top of the rest of the hair.
You really don't see many predominantly whites either as most animals these days are moasics, which is any broken pattern - a spot of grey anywhere, etc. Predominantly white is just that - white with no grey, black, silver, etc.