Would you consider simply coloring the particles on the arrows that she fires Black? Both the arrows that she is holding and the arrows in her quiver are a steely black.
In fact, I for one think the icy particles fight against the amazing concept of this skin: A darker themed Ashe who has finally turned to whatever means necessary to make sure her people are safe, even teaming up with the notorious werewolf.
I have recently been very impressed with the strong thematic elements of riot's recent skins, such as El Tigre Braum and Firecracker Jinx, and I am very happy that Ashe has been chosen to represent some of this new, more deliberate tier of skins. Therefore, I don't think you should hold yourself back in making this skin as true to its vision as possible, without regard to its price tier.
I for one, as a dedicated Ashe player, am thoroughly impressed with this skin, as it fulfills a fantasy I have been waiting for in a dark-themed Ashe skin, and I would love to see that theme carried over into her arrow particles: it would be as if her armor and choices have truly tainted her, like the black frost on Anivia.
Additionally, she's finally wearing clothing armor! :D I'm, very excited. I could even see this skin having voice processing because of the helmet she is wearing, and would have no qualm about it a change in price tier simply because I know this theme was pursued to its greatest extent on Ashe, a champion that I am unfortunately a fan of despite her relative 'niche' among marksmen.
TLDR: I love this theme, and am thankful that Ashe is getting more love. For that reason, however, please don't stop yourself from making this theme as strong as it can be on her. I think many would be extraordinarily pleased with a change to Ashe's arrow particles, even it is a simple cosmetic change such as with Amethyst, and would not qualm about a price.
Once again, thank you for the Ashe love.