Thanks for your reply.

The (original color) pink and (new color) red come off the same branch which splits off mid-air.

Is "red sport" a variety that produces flowers with different colors?

THen it is most likely a sport. Here's an explanation of a sport on WIki

https://en.wikipedia.org/wiki/Sport_(botany)

Shima NIshiki is a sport of Taiyo Japanese Tree Peony. IT has white and red striped flowers. If your red flower is super special, you can propagate the sport by doing grafts with them. A sport is definitely a random mutation .

It is a normal Shima Nishiki. The flowers will vary each year. I have had them be white with a blush of pink, red, and red and white on the same plant. I have also had them one solid color one year and the next year a totally different solid color.

If your cultivar is “Shima Nishiki”, it is common. If not, it will be a mutation.

The usual thing seen is a tree peony grafted on an herbaceous peony, which has unknowingly been allowed to sprout and produce its own flowers. Otherwise one English approximation for nishiki is brocade, with plants of various kinds so named having variegations or broken color patterns.