Hi! ♥ This is a really interesting question! Because there’s a lot of complicated stuff going on here! We really only know for certain that there are three Silver Millennium people: Queen Serenity, Princess Serenity/Usagi, and Chibi-Usa/Small Lady.
Queen Serenity has silver hair in every instance we see her, but we only ever see her in her Queen Serenity form, we never see her previous to that, if she had a Princess Serenity form or if she had some other form.
Usagi has many forms–Princess Serenity (silver hair), Tsukino Usagi (blonde hair), Sailor Moon (blonde hair), and Neo-Queen Serenity (silver hair). It gets complicated because the anime series have changed her Princess Serenity form to blonde hair, but in the manga it’s meant to be silver.
Chibi-Usa is only ever shown with pink hair, whether in her civilian form, her Sailor Soldier form, or her Princess Small Lady Serenity form. Both before and after her Sailor Crystal becomes the Pink Moon Crystal, she has pink hair in both forms.
So, we really only have three examples and none of them are consistent enough to be able to tell anything. Queen Serenity and Princess Serenity were practically clones of each other (which often brings forth speculation that Princess Serenity wasn’t born as part of a union between Queen Serenity and someone else, but that perhaps Queen Serenity split her Star Seed and created her daughter that way, hence the identical looks) and Chibi-Usa is a totally brand new thing, there’s never been an Earth People/Moon People child before.
Then you’ve got Chibichibi, who can be speculated to be part of the Silver Millennium line and I would actually agree that that’s pretty damn likely, because she’s some future Sailor Moon (just like Chibi-Usa is a future Sailor Moon) and she has red hair in her civilian form and silver hair in her Sailor Soldier form. We have no idea if she has a Serenity form, but I would venture that it’s probably pretty likely, too. (I go with the far future descendant theory, especially because I like the idea that that’s where the pink/red hair comes from!)
So, I like the idea that silver hair is associated with the Serenity forms, but I think… in Sailor Moon, so much is about self-identity and how your heart works. The entire point of Chibi-Usa’s story is about how she sees herself, that’s why she gets a new Serenity dress and why her Sailor Crystal becomes the Pink Moon Crystal: because now Chibi-Usa is her own person.
It’s hammered into us many times that the Silver Crystal works based on what’s in Usagi’s heart and I have the tentative theory that her hair is a reflection of this as well. Like….
Princess Serenity & Neo-Queen Serenity - silver hair, because that form is associated with Queen Serenity’s kingdom, she remembers Silver Millennium on the moon, she remembers her mother, she remembers everything from her past life. When she becomes Neo-Queen Serenity, that’s just how she sees herself in that form, as part of the legacy of the Moon Kingdom.
Tsukino Usagi & Sailor Moon - blonde hair, because they’re less associated with Silver Millennium, because she doesn’t have those memories for a long time in those forms, because they’re unique to just her. And I like the theory that, once she went through the Galaxy Cauldron and came back to Earth, once she chose this planet to protect, her hair reflected her new heart’s association. The moon is silver, the Earth is golden. When she decided that she wanted to live here, when she wanted to protect Earth, when this was her home, her hair reflected that about her. (And I think of gold as associated with the Earth, like with the Golden Crystal = Earth. Same for the Moon Crystal = Silver coloring.)
I think that Usagi patterns her Serenity form after her mother and her Sailor Soldier form based on her individual wants, that’s just the way she works. Chibi-Usa patterns her Serenity form on her mother but also on her own individual self, because that’s the emotional journey she goes through. Now, maybe when she ascends to the throne as Queen Small Lady Serenity, her hair will turn silver, but I don’t think so. Princess Serenity always had silver hair, that didn’t change when she ascended, so I don’t think there’s any reason to assume Princess Small Lady Serenity’s hair would change… not unless she wants it to.
Basically: I think their hair color is based on what’s in their heart, more than that there’s a trend in the Silver Millennium family lineage. ♥