#kuitou

Hi! Thanks for the question, and sorry for taking ages to reply!

The pom poms you see on 盔头/kuitou (Chinese opera headdresses) are called 绒球/rongqiu (lit. "velvet ball"). They are often red, but can also be other colors, and vary in size. Rongqiu are decorative and serve to distinguish the many different types of kuitou from one another. Each type of kuitou is distinct in the number, size, and color of rongqiu that it's decorated with (of course, not all kuitou have rongqiu).

Below - a few different types of Beijing opera kuitou decorated with rongqiu (x):

Rongqiu isn't used just for Chinese opera performances - it's a very common decorative item for Chinese headwear, especially for traditional/folk performances.

Below - examples of rongqiu use in folk custom/performance costumes, left to right: 1) 游神/youshen (wandering gods) procession in Fujian (x), 2) 英歌舞/yingge wu (yingge dance) performer in Guangdong (x), 3) & 4) 高跷/gaoqiao (stilt walking) performers in a 社火/shehuo parade in Gansu (x):

As a festive decoration, rongqiu was also widely used on bridal guan (crowns) from the Qing dynasty into the modern day.

Below - examples of rongqiu use in historical bridal guan: Left - a bride during the late Qing dynasty, circa 1890 (x); Right - a bride during the Republican era/minguo, in 1939 (x):

For some reason it's been extremely difficult to find sources on the origin of rongqiu that would shed more light on its significance, but based on historical paintings the use of rongqiu as a head ornament may have originated in the Qing dynasty. During the late Qing dynasty, it was fashionable among women to wear rongqiu on the sides of their hair, as can be seen in the paintings below (x):

This particular style of rongqiu hair ornament was depicted in the 2012 historical cdrama 娘心计/Mother's Scheme:

For more references, please see my rongqiu and kuitou tags.

If anyone has more information on the significance of rongqiu, please do share!