I was originally going to go with red as the colour of demons in my latest work: They appear through rifts in reality that manifest as a red haze that falls over everything: A confusing, eye-hurting red in which barely-visible monsters lurk.
But the main good guys of my setting, um, use red as a symbol. Their blades are red, their ethnic group has impossibly crimson-coloured hair, and their outfits are red and black. The idea is that they associate red with life and vitality, because blood is red.
I considered going with purple or magenta for the demons, but those didn't really feel elegant in the prose.
"Something moved in the magenta. Something awful, whose shape made no sense but radiated violence. He fired at it, but wasn't sure if he hit. The thing darted to the side, vanishing into that hideous magenta..."
I then considered making reference to an awful pink colour, but I don't know if that works either.
My latest idea is to make it a lurid blue colour. There would be a coldness and a darkness to it.
I need to figure this out pretty fast, and I don't know why on Earth I've taken so long to make this change. And I would really appreciate some opinions.