
It is possible that your daylilies could be TC (tissue culture). TC plant material often reverts back to one of the parents if a hybrid. For example, Hosta Patriot will "throw" a solid leaf instead of the variegated it is supposed to be. Hopefully this is what has happened to your daylilies, and future blooms will be their correct color.
Another possibility could be that a H. Stella 'd Oro set seed in the past. Those seed could have germinated in the clumps of red, pink and garnet daylilies, and are blooming for the first time this year.