Steve : He looks impressive for sure.................. as for the coloring it's looks like some sort of infusion somewhere down the parent lines, it apears to only be a smattering at the end of the feathers so the pin's should still be good and white. If you are concerned with it you could cull against it, but if it's a non issue heck leave it be and it still may never be passed along.
You also said quote.... I've seen older Dels or Leghorns get yellowing with age , but I think this looks different. And......If you look close there is also an occassional tick of color in the white , are any of these deviations from pure white an indication of silver ?
Yeah that yellowing you speak of is just common corn fade, if the birds without a true silver gene have any corn in their diets over time they will fade to a slight yellow with age, true silver whites never ever can fade. Now Steve that ticking along that flight feather isn't a tick, by a tick or ticking I mean 1 single small under down feather, your markings are many along one feather not the same, and sometimes Steve these could disappear with your birds first molt and never return.
AL