Whites can cover anything. Black is a completely separate gene than blue. While bl is commonly referred to as black, it is more correctly termed "not-blue."
All chickens have some form of the black gene E. With silkies it is typically e^b. Black silkies result from e^b plus melanizers, not from E, unfortunately. (E would make it easier to create solid coloured birds.)
Pg is very common in silkies of all colours, and will give concentric penciling to female feathers: partridge, especially on an e^b base.
Grey is an entirely separate colour from blue, both genetically and in appearance, so calling splash a mix of dark & light grey is confusing. A better description is a lighter bluish base colour with uneven streaks of dark blue. Splash is the result of two copies of the blue gene; all offspring will receive a copy of the gene, and therefore display blue in some form, unless hidden by other genes (such as recessive white).
The completely naked neck and bowtied showgirls indicate two and one copy of the naked neck gene, Na, respectively. Those with one copy will only pass the trait on to half their offspring; those with two copies will pass it on to all their offspring.
White turns OFF all pigments, so that whatever colour the bird is genetically does nolt show in hte plumage. The white in silkies is typically recessive white, meaning that a white silkie has two copies of the gene and will pass a copy to all offspring. Therefore breeding white to white should always yield white. As with all genetics, sports (genetically unexpected results) occur occasionally, so very rarely two whites will produce some other colour.
Because white hides the other colours that are genetically present there is no way to predict the colour of offspring when bred to a different colour. The only way you will get splash is if the white is hiding blue, which is possible, but don't count on it. If you do get splash, then you will know that your white is carrying blue.
Lavender is recessive, and therefore unless the bird carries two copies, it doesn't show in the plumage; this is what split is: one copy of a recessive gene.
A bird can be both blue and lavender,and will display intermediate to both. Generally a lighter blue, but the head, hackles & saddles will still be darker--its appearance is more live blue than like lav ender, even with two copies of lav.