I was talking more on genetics. I thought his grandfather's hair is white because he's old. And Silva's hair looks more blond than white in the cover. Even if his hair is white, is the gene for white hair super recessive that only Killua was able to inherit it? I just thought that, with all the research Togashi does for HxH, he could have thought of gene inheritance too. Oh well, I actually forget about our hereditary lessons in biology, so I may just be rambling nonsense here. It could be possible that Killua is the only one who inherited white hair. Or whatever.
When it comes to heredity, there are recessive and dominant genes. Each offspring receives a "version" of a specific gene (dominant or recessive) from each parent when they're made. For a single gene, which will also determine the appearance of the offspring, this will come in 3 varieties: XX (where capitalized "X" is the dominant gene), Xx (where the lowercase "x" is the recessive gene, and "X" is the dominant gene), and xx (where "x" is, again, the recessive gene).
Although the recessive version may be present in the Xx variety, because the dominant version is there, the dominant version is the one that will be displayed.
So, supposing X = black hair (the dominant gene) in the Zoldyck family, and x = white/lighter hair (recessive).
If we're going to characterize Silva's genes, he would require "xx" since he needs two recessive genes to get his lighter colored hair.
I'm going to assume Kikyo has dark hair, but based on Killua's appearance, we know she also carries the recessive version with her since he has light hair. So I'm going to characterize her genes as "Xx".
Now if I match up Kikyo and Silva's genetics to get the probability of a light-haired kid showing up, I get these four genes: Xx, Xx, xx, xx.
This means there's a 50% chance of a kid with light hair showing up. The other ones have black hair.
If no one cares or this doesn't make sense to anyone, I apologize.
/Biology geekdom
Of course, I doubt Togashi thought of it that in depth. According to his profile, I thought Silva has silver hair and blue eyes, and so does Killua. It's probably to reiterate the fact that out of all his siblings, Killua is unique and therefore the named heir of the Zoldycks. I'm going to assume the other siblings take after Kikyo's appearance in some way or another, but I still see characteristics of Silva in the older siblings as well (i.e. height). I'm also speculating that by the time Killua reaches Milluki or Illumi's age, he's going to be pretty tall.
