The story in question is quite dark, so I thought I'd try asking this group.
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; ponies would refuse to touch them. At about the middle point of the story, an infant is mercy-killed because her aura was an unprecedented bright red, like Sombra's, and the parents knew she'd never live a normal life. Does this sound familiar to anyone?