Why does only Geralt and Vesemir have white hair?
I'm surprised that this is the first time I actually been asked this question about Geralt's iconic white hair. His iconic white hair was due to a result of an additional mutation he went through during his training.
The Witcher season 2 proves that white hair and yellow eyes are not a characterizing Witcher trait, since they can be attributed to the extra mutations that Geralt is subjected to as a child.
Geralt is a result of his exposure to further mutation, due to his unique tolerance for the Trial of Grasses; this extended process left his hair devoid of pigment and possibly left him even stronger and faster than his brethren. How this process was developed is a mystery even to the Witchers themselves.
Sadly, only four out of ten boys survived the trial of the grasses and became a witcher. Like Vesemir, Geralt, Lambert, Eskel, and other witchers, Ciri never underwent the trial of grasses.
Geralt took to the mutations to make him a Witcher better than anyone else, so because of this success they exposed him to even MORE mutations than a typical Witcher. These additional mutations caused his hair to lose all of it's pigment and turn white, which is what has led to his nickname The White Wolf.
4/10 Geralt of Rivia Is The Strongest Witcher
Not only is Geralt an incredible swordsman, but he also demonstrates a strong proficiency with magic as well, using the witcher Signs to great advantage in battle.
Ciri's white hair is a result of genetics. Both her mother and grandmother have white(silver) hair, explicitly described in the books.
Ciri has Ashen hair because she was born like that. Ciri's ashen-flaxen hair is genetics into play. Is a trait in her bloodline same as her green emerald eyes. Geralt's white hair is because he was subjected to experimental mutations which made him loose his natural hair pigmentation.
Are there any female Witchers in the video games/novels? According to a report from TheGamer, it's mentioned in The Witcher novels that there were women who tried to become Witchers, but none survived all of the trials. However, there are no actual full-fledged female Witchers in the novels or video games either.
Anya Chalotra, who plays Yennefer had beautiful purple eyes. The contacts that change the color of darker eyes, like Chalotra's, are much thicker than the ones that can be used to change light eyes. They used a combination of contacts and CGI to make her eyes purple.
Is Geralt special among Witchers?
Geralt, played by Henry Cavill in the series, is considered a unique Witcher. Because of his ability to withstand the Trial of the Grasses, he was subjected to additional testing, causing him to gain more abilities while also turning his skin and hair pale white.
Because they went further with Geralt during the Trial of Grasses. Seeing that Geralt was very compatible with the concoctions, Vesemir and the rest decided to mutate him more than normal witchers, and as a result he has greater physical abilities, but not by much.

During the Trial of the Grasses, Geralt exhibited unusual tolerance for the mutagens that grant witchers their abilities. Accordingly, Geralt was subjected to further experimental mutagens which rendered his hair white and may have given him greater speed, strength, and stamina than his fellow witchers.
Ciri is by far the most powerful being in The Witcher universe and not many villains can come close to what she's capable of, particularly her video game version. She's not exactly a mutated witcher but her time and space manipulation powers are more than enough to put her in an even ground against Geralt.
What makes Ciri so special and powerful is that she processes Elder Blood and this comes from her descendant, Lara Dorren, an elven sorceress. Ciri's Elder Blood is also known as Hen Ichaer or Lara gene, an incredibly rare heritage passed down from Dorren.
Yennefer of Vengerberg
In book/video game lore, however, she mentions being 94 and is usually considered to be a few years older than Geralt.
She's learned that Ciri is part elf, the child of Elder Blood foretold in an ancient elven curse meant to bring vengeance upon the humans.
Due to the extremely taxing nature of the trials, the majority of individuals die before they can even complete the Trial of the Grasses, regardless of gender. The aforementioned 30% that are able to withstand this initial trial are always boys; no girls have been able to do so, though there have been attempts.
Geralt was a special case, as he showed exceptional tolerance to the experiments and mutagens, and was subjected to more tests. All these processes left Geralt with incredibly pale skin and white hair, earning the name “Gwynbleidd”, which in Elder Speech means “White Wolf”.
Although there are few people without some sort of elven blood in the world of The Witcher, some races are still considered to be "non-human". These include: Dryads.
Is Ciri more powerful than Yennefer?
Ciri. Ciri is the de facto strongest character in the Witcher. Even though you might think the book is mostly about Geralt, or maybe even Yennefer, in reality the whole story is about Ciri.
We know that in Geralt's times, he is the strongest, most skilled, and most successful Witcher. This is because the average witcher is weaker and not as skilled as Geralt, due to Geralt receiving extra mutations and in general being more effective, going toe to toe with some of the deadliest creatures known to man.
Ciri hasn't undergone any mutations. She trained as a witcher when she was young, but never underwent the mutations, so her swordplay is not bad. She just lacks the superhuman reflexes, speed and senses that the mutations give witchers.
"Płotka" is the Polish term for a female horse, and it's also an informal version of the word "płoć," which literally translates to “roach” or “roachy.” Geralt is known for favoring mares over stallions, therefore, the monster hunter's preference ties his steed's name to Polish terminology.
Ciri has Elder Blood because a certain elven sorceress named Lara Dorren fell in love with a human mage named Cregennan of Lod. Their daughter, later adopted by Cerro, the queen of Redania, was named Riannon. Ciri is the descendant of Riannon. Ciri's Elder Blood is another reason the King of the Wild Hunt is after her.