Grandmaster Voryn Dagoth in mourning after death of Wulfharth of Atmora (who later ended up being brought back to life)
headcanon: the Chimer had two colours associated with mourning - red and grey. Grey, associated with ash and ceremonial burial, meant peaceful mourning, acceptance of the loss and calm, quiet contemplation on mortality and the veil between the worlds of the living and the dead. Red, on the other hand, expressed despair, pain, vengeful mourning, and was typically worn by families of those murdered or executed. It was not the “dignified” way of grieving, and for the Grandmaster of a Great House to show up in an all red outfit - parts of which were made from a kind of a poisonous giant moth - for a council meeting with the Hortator was rather bold.
After being changed into Dunmer, the colours mostly lost their significance; however, some continued to see grey and red as the colours of death and mourning, and from their eyes and their skin they made a symbol of grief for the lost glory of Resdayn and before it the High Velothi culture.
In 3th Era, the newly re-emerged Sixth House adopted red as their colour, corresponding with their self-given title of Tribe Unmourned.