It's been forever since i've read Past Sins, So I've kinda forgotten what the color of Nyx's magic is. SOme pictures show it white, others blue, i even see a black here and there. does anyone remember the actual color?

i think its cobalt blue

3011187 thanks!

I could have sworn it was indigo, but I could be wrong.

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I remember it being said that her magic colour is indigo.

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But then again, I am always right.

In all seriousness, I think it changes from author to author. If it was actually mentioned in Past Sins or another of Pen Stroke's Nyx stories, it's one of those details no-one remembers or cares to look up.

3011323 isn't cobolt a kind of indigo/blue?

3011369 I red 'Past Since' by Pen, I enjoyed it, all the way.
Even if Nyx was made to become Nightmare, there are several important differences,
nyx suck at painting the sky, as I recall.
So mybe it isn't the same colour of magic either?
I'm building a story with the asumption you could tell ponies apart by the colour of their individual magic.
even if most ponies owuldn't see it enough to be sure in all cases.

I also think it is indigo like Luna's. I wonder how close their DNA is. Would Luna and Nyx be mother and daughter close or clone close?

Well. I though this would be trivial to clear up. There's a few chapters in Past Sins in which Nyx uses her magic extensively. Any time anyone describes a pony using magic it's, "a magenta glow enveloped the cup," or some such. Not in Past Sins, apparently.

So, yeah. I envision Nyx's glow to be some sort of deep purple/violet, similar to her mane color. I was expecting the source material to back me up on this, but no dice.

I just tried a word search within Past Sins, using several different keywords.
Doesn't seem like Penstroke ever mentions the magic color of any unicorns.

He does describe Nyx's natural eyes as:

brilliant turqoise irises and whites that were a lighter teal

And as Nightmare Moon her mane is "indigo".

So I guess you can either make up whatever you want, or never mention a specific color.

My personal opinion
I believe her's isn't Indigo as that's Princess Lunas, instead I think it is that sharp kind of dark Blue like Nightmares as that was where her original power came from.

White isn't really kind of her style and other than that Magic tends to follow color asspects of its user I.E. Twilights is a Magenta Lavender depending on her artist.
So I believe if not blue the Turquoise or black, also Purple like her mane

I'd say that it's noticeably darker then Luna's but not by much

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Nightmare's magic aura is cobalt when it's inside Luna's and Rarity's bodies, though.