We had no idea what changes to the story they have made.  It's quite common for movie/TV adaptations to to deviate on many elements from the source material. Be that video games, comics, etc...   Why they decided that is something they should explain and it's almost certainly not just they didn't want to cast a white girl for that one role.  There will be some sense to it.   But it's not information we will get from a casting call. 

Is Ciri's skin color relevant to the story of Witcher?

I've just explained that in few posts above - she doesn't have to be white. But if you change her skin, you're changing whole Cintra, which changes lore. Which could be sort of fine, in theory...except, Cintra is located where it is and there's about zero logical sense for that. Technically, you can start changing kingdoms south of Cintra as well, changing the lore even more...until you get all the way to Nilfgaard...which is white...so you have to change them as well, breaking the lore even more. Now, as a side effect, you have main evil guys in Witcher being non-white...oh dear.

As I said already, there is reason why you don't pull shit like this - Witcher is based on Medieval Europe, with rich lore taken from it:

Nilfgaard mostly resembles Holy Roman Empire under Habsburgs.
Redania is most likely Prussia.
Temeria is argued to be Polland itself.
Novigrad is Gdansk/Danzig.
Toussaint resembles Southern France mixed with Tuscany.
Skellige is mix of Nordic and Celtic culture (it is located across the sea northwest of Cintra).

Then you have:
Zerrikania (to the southeast of Temeria, long way across Korath Desert) seems most likely as mix of Persia and India.
Ofir (way south of Nilfgaard) which mostly resembles Ottomans mixed with Northern Africa.

Part 1, yes I get what you are saying, what you don't get is that first identity policies is more an issue than a problem solving, second as much as there are 3rd gen japanese in USA who speak english there are also 3rd gen european in Japan who would talk Japanese. And also that no one have a right for a role or quota. Also if you are making a current story sure there are space for most folks who migrated there. Still if you are talking a story about a period in time that only England migrants had anything to do with the story but then you decide to make the queen of England black on a USA Independence story then you are taking silly liberties just to be inclusive "why can't this 10th gen african american, who is american through and through be the queen of England?".

There is no push for more roles for indian, japanese, russian, arabic or european descendents people in Brazil to have space on the shows (and we do have a good bunch of migrants over here), it is usually the "give black people voice".

In USA it is a little more broad, but we won't see similar things in India or Japan even if there are plenty of white folks over there who can act. There may be an actor or another who get a role, but I seriously doubt there are major groups pushing and demanding that more roles are given to these people over there. As much as 3rd wave feminists love to preach about all the perceived inequality in USA complain about christian and patriarch at the same time demanding recognition to islan even if the countries where that is majoritary woman and gay are threaded much worse than USA.

I'm pretty sure I can name more black or indian actors in Holywood than you would be able to name white actors in India, Japan or let's say Africa.

It is quite relevant to lore of Witcher and how world of Witcher is set-up. Change that, and you have chain reaction that affects big part of that set-up...which is technically possible, it's just that isn't Witcher that Sapkowski made anymore.

Personally, I don't think it matters what all of these places are based on. As I said before, fantasy is about creation of culture. If a change in skin color destroys your story, the problem likely lies in your story.

That said, the overall geography of the land is a fair point to make. I would be interested to see how this would be handled overall, but I don't think having a few regions being a bit more tan would shatter the coherency of the story. Even if a geneticist or geneologist would insist that there would be more gene flow between groups causing skin colors to not diverge so much, I think that would hardly be a very large issue with the story. It is a bit hard for me to argue the specifics of this not knowing the lore of the Witcher very well, but to liken things to Game of Thrones, I wouldn't think anything of it if the Ironborn were non-white. I don't think it is very hard to write different regions of your map to be physically different in a way which would make sense to the viewer.

Now, I would agree that this may cause some issues with the interplay between the books and the adaptation, but I don't think it is the job of an adaptation to follow the source material religiously. It is their job to make a good show. I mean, take The Walking Dead for instance. From my understanding of fan reactions, its best season was season one , which was largely original material, not following the comics and its best character has pretty much always been Daryl who isn't in the comics.

It all comes down to execution.

Certainly doesn't imbue me with much confidence. That and what happened to Death Note when Netflix adapted it.

Let's go back to point 1.

1. But that's irrelevant in a casting call.  The choice of the character and what they should or should not look like has already been determined.  It's a casting call.  Not a story development meeting.  That's already happened.

To expand further.  The choice of this character now having darker skin has been finalized and the explanation for such is not going to be given at a casting call.  The fans may want an explanation but a casting call is not the venue they will receive it from. It just isn't.  If it bothers you, contact the production company and ask that they provide an explanation for the change.

Personally, I don't think it matters what all of these places are based on. As I said before, fantasy is about creation of culture. If a change in skin color destroys your story, the problem likely lies in your story.

That said, the overall geography of the land is a fair point to make. I would be interested to see how this would be handled overall, but I don't think having a few regions being a bit more tan would shatter the coherency of the story. Even if a geneticist or geneologist would insist that there would be more gene flow between groups causing skin colors to not diverge so much, I think that would hardly be a very large issue with the story. It is a bit hard for me to argue the specifics of this not knowing the lore of the Witcher very well, but to liken things to Game of Thrones, I wouldn't think anything of it if the Ironborn were non-white. I don't think it is very hard to write different regions of your map to be physically different in a way which would make sense to the viewer.

Now, I would agree that this may cause some issues with the interplay between the books and the adaptation, but I don't think it is the job of an adaptation to follow the source material religiously. It is their job to make a good show. I mean, take The Walking Dead for instance. From my understanding of fan reactions, its best season was season one , which was largely original material, not following the comics and its best character has pretty much always been Daryl who isn't in the comics.

It all comes down to execution.

You say there are third generation whites in Japan:

98.5% of Japan is ethnic Japanese. 0.5% are Korean, 0.4% are Chinese. That leaves 0.6% making up literally everybody else (which is less that 1million people), and a large portion of these are individuals who came to Japan attached to some pre-existing project, such as individuals who came over through their company. Again, show me that there is an underserved population of actors in these areas, and I would gladly support greater diversity in their cultural products, but again, it is not the place of Americans to demand change in Japan (in this context).

It is not hypocrisy to demand change in your own culture while not demanding it in someone else's.

Everytime, the issue is not the skin or gender or hair color itself, but those decisions are just representative of how few f*cks the directors give about the shows, and it's usually obvious when you watch it.

Sort of like in Preacher.  They changed a white, blonde lady from the comics and made her black for the show.

Yet for three seasons, the show has been praised by critics and audiences alike, according to RT, despite deviations from the source material. 

It is hard to be convinced that the issue is”not the skin or gender” when people keep losing their minds over skin and gender.