@alkalische, thank you for your tags! I actually have an adjacent headcanon about Ines and red.
I believe that when she uses her Arts, the colours around her fade and the shadows are amplified. As a result, while her vision doesn't get completely black and white, it gets pretty close to it.
However, after she started living among the Sarkaz, she conditioned herself to decipher colour red. Now her "Arts vision" looks kinda like Darknights Memoir cover (I'm not gifing it, but you know the figures in the back are all shadow-y):
Ines uses red threads because she can easily distinguish them among the shadows. This is also my explanation as to why W's DM design and Hoederer's third evolution have her threads: it helps her to spot them on the battlefield. In addition, we know that using her Arts can affect her eyes, so it may double as a vision aid.
I also think it partially affects her perception of people. Among her friends, Hoederer and Ascalon are both redheads, W has red horns and both she and Paprika wear outfits with prominent red details. Meanwhile, Theresa is one of the people she doesn't want to use her Arts on, and when she first saw her she described her as being "white" (as opposed to "shadowy black" I presume she usually sees).
