This looks like a good group to ask. It was a story that depicted a grim cultural belief among unicorns that the colour of a unicorn's aura was critically important and the pony could be judged and treated differently based upon it. The other colours varied in their rank and treatment, but the worst treated was the aura they called 'Gules', if I'm spelling that correctly. These unicorns were pariahs, ostracized entirely. Does this sound familiar to anyone?
I've been asking a few different groups, and I'll probably ask a couple more Grimdark groups. I've been back and forth through Google and through the FimFiction search engine, and I'm beginning to worry that the story was taken down.