One addition: Amarië is a Vanya, so presuming her to be blond is fairly reasonable, though of course we don't have to go that way.
Personally, I would picture Nerdanel as having brown hair with red highlights, but I recognize that there is some leeway with her.
I'm sorry to butt in without having listened to the podcasts yet, but I have something constructive to add: we do know what Nerdanel looks like.
The Shibboleth of Fëanor notes that the first and last of her children had "the reddish hair" or "the red-brown hair" of Nerdanel's kin. It also notes that Caranthir "was dark (brown) haired,
but had the ruddy complexion of his mother" (my emphasis). According to these inferences, she should have ruddy skin but a different hair color than Caranthir's dark (brown) hair.
Just above, he had written that Caranthir had
black hair. (Tolkien also writes about Elvish hair colors here and there in
Parma Eldalamberon 17, where he wavers on whether Elves ever had
black hair, or merely dark brown). Despite this, note 61 on Nerdanel's father says "His hair was not as dark or black as was that of most of the Noldor, but brown, and had glints of coppery-red in it."
The Problem of Ros then notes that the "red, red-brown hair" of those sons was inherited "from their
maternal grandfather" (my emphasis).
With all of these pieces together, I would expect that Nerdanel had ruddy skin (no question), but did not share Caranthir's hair color (whether black or dark brown). Additionally, if those sons inherited red or red-brown hair from her father, she would not have it herself. If black, dark brown, and red / red-brown are eliminated, she is left with silver (Telerin heritage is not mentioned), blonde (Vanyarin heritage is not mentioned), or brown.
Christopher Tolkien mentions that there were "many experimental etymological notes on the Eldarin words referring to red colour and copper, and on the names of the twin brothers, which are here omitted", but they have been presented in
Vinyar Tengwar 41 and confirm that Nerdanel had "brown hair and a ruddy complexion".
Regarding Amarië,
Quendi & Eldar notes that "
nearly all members" of the Vanyar (my emphasis) had yellow or deep-golden hair. So there's some leeway, and a range of different shades of blond.
As mentioned, rule-bending is possible, though there is some more canon to dig up if desired (including the Sindar and occasionally the Valar).