This is Indian Paintbrush. I have never seen this sort of colouring on a daylily. How unusual is it?

I'm not sure what causes it but I some of my daylilies are prone to getting strange stripes of a different color on the blooms from time to time. Typically mine aren't so uniform as that but I have seen it. Holly Hill Tangelo is prone and this year so is Bourbon and Branch. It isn't every flower so it probably isn't genetic. Must be something environmental that causes it but I don't know what it is or why some daylilies are more susceptible.

If the question is about the light stripe down the middle of each petal, that is not unusual, see "midrib" in the Daylily Dictionary https://daylilies.org/daylily-...

I think it was about half of two petals being a different color.

Sometimes daylily flowers do not open all the way and half (or some other fraction) of the flower gets caught up. When that happens the sunlight does not shine on the hidden part (shaded part) of the petal. The colours of the two parts of the petal may be noticeably different.