Cassie answers a very debated question on Tumblr

Valentine's hair color and style.  Such a debated topic. 

"Coatnayy" asks Cassie on Tumblr. 

“I’m not sure if anyone’s asked, probably has, but do you know why they didnt choose to make Valentine’s hair fair/almost white I’m the adaptation? I imagine it would’ve been hard to find someone who could match that or pull it off, but it just ties in to the stories so much to me that I was shocked when I saw dark hair. I’m afraid it might make Johnathon/Sebastian less interesting in the (hopeful) movies to come.
— coatnayy”

Here's what Cassie had to say.

—

I’ve no idea, really, no more than I know the answer to the lot of questions about Jamie’s eye color or Lily’s hair color or Isabelle’s height or Alec’s sleeves — I am absolutely not involved in these films on such a micro level that I know about wardrobe decisions, and therefore I didn’t ask about them, since annoying everyone about micro issues is a good way to make sure you get left out of decisions made about macro ones. I think it would have been a mistake to pick the actor for Valentine based on their hair color. On the other hand, I am not going to pretend I don’t know Valentine doesn’t look the way he’s described in the books, clothing and hair-wise. :) I don’t know if hair dying wasn’t possible, in the way wearing colored contacts is often not possible for actors (people tend to talk about it like it’s a simple thing to just pop them in and actors are terrible lazy people if they don’t do it, but they irritate a lot of actors’ eyes and limit their peripheral vision which can be problematic for action scene.) In the end, in 99% of cases if your book is being adapted into a film, you have to let it go and accept that what you’re going to see onscreen is an interpretation of your books. Film is a visual medium that provides single definitive pictures while books rely on reader imagination to fill in those pictures and allows for variance. Changes that don’t seem to have bothered people: Harry doesn’t have green eyes in the HP films, nor does Hermione have buck teeth; Legolas is blond, not dark-haired as in canon, and Frodo is nineteen, not fifty-something as in the books, in the LOTR films. Some changes/alterations work (casting Frodo so young works to make him a visual picture of ruined innocence as the Ring destroys him and his delicate youthfulness crumbles into wreckage) and some don’t. There’s no way to know until you see the film. As for Jonathan/Sebastian, I think as long as we know in the first film that Jonathan Morgenstern/Jace/Clary’s brother was/is blond, I am not sure why his dad’s hair color makes a difference to his character otherwise? I wouldn’t want to lose the dyed hair/disguise from City of Glass but that doesn’t require anything particular from Valentine.

Source

Why do you think Jonathan Rhys Meyers hair wasn't dyed?