I'd think it would be more fair if Blood Elves got the Dark Ranger/San'layn NPC skin and eyes as options rather than arbitrarily denying Void Elves any color options in the Human spectrum beyond black and white. I prefer more options for everyone, to gating options for specific races.
I do think that Blizzard should share Void Elf hairstyles with Blood Elves, even if they are modified versions, as a sort of recompense for the sharing of skin tones. The hairstyle I use for my character (as seen in my avatar image), is exceedingly popular and I know of several Blood Elf players who'd like to have it. Overall though, I think Blood Elves need more options period. Compared to other core races, Blood Elves really feel like they got shafted.
I'm not sure how Blizzard intends to handle customization for allied races when their time comes. I would expect them to take the path of least effort and simply have allied races inherit straight up copies or modified versions of options from their parent races wherever possible. Several Void Elf hairstyles are already modified versions of Blood Elf styles so there does seem to be a precedent for that. As far as hair colors go though, I'm hoping that Blizzard doesn't just copy the Blood Elf hair colors to Void Elves. It would add salt to the wound that some Blood Elf players felt after having shared their skin tones, and it would make Void Elves look virtually indistinguishable from Blood Elves. While some players would like that, I would not.
At the same time, I don't think it unfair for Void Elves to get at least a few natural human hair colors so long as they aren't identical to the options Blood Elves have or as numerous. As I linked earlier, I think these colors would be fine for Void Elves. And even if that's all they got as far as hair colors go, it would do wonders for appeasing not only those seeking the High Elf look, but also those who simply want more variety. Maybe they want to be portray a half elf that joined the Ren'dorei (I'll be shocked if Void Elves don't get the same ear size options that Blood Elves currently have), maybe they just like the way a particular hair color looks with the glowy tentacles. Void Elves should not be stuck with only hair colors within a given spectrum while everyone else gets a wide range of hair colors.
While this is not as huge a selection of colors as Blood Elves have, it covers the basics while remaining relatively distinct from Blood Elves. Having a single dull beige blonde option that's nothing like any blonde that Blood Elves have isn't an unfair ask in my opinion. It's a somewhat darker version of the blonde option that Humans have and nothing like the variety of blondes that Blood Elves possess. No wide selection of bright platinum or lemon yellow blondes, just one diffused beige option.
As I mentioned above, I do think that Blood Elves should get the Dark Ranger/San'layn skin and eyes so they can have their own "secondary theme" as some on the official forums like to say. On top of that, I don't think Blizzard should allow Void Elves to inherit the distinctly Silvermoon themed jewelry. If Void Elves get any jewelry options, I feel they should be more in the style and color scheme of their heritage armor, though personally, I'd be fine if they didn't add any jewelry options to Void Elves at all.
When it comes down to it though, I think that all of the core races that are still lacking, including Blood Elves, need to have their options fleshed out further so as to be as satisfying as the Night Elf and Human options are, before allied races in general, and Void Elves in particular, get anything added to them. Once the core races have a satisfying selection of options, then the allied races can get their turn. And when that happens, the very first allied race that should get focused on is the Nightborne in my opinion. Of all the allied races they need love the most!