The rough hemp cloth worn by the children at a Chinese funeral (披麻戴孝) is uncomfortable on purpose. Those closest to the deceased are meant to bear the most.
In ancient times, mourning dress was divided into five grades (五服). The closer you were to the deceased, the coarser the hemp and the longer the mourning. The heaviest grade, worn by children for their parents, used unhemmed cloth and meant three years of mourning.
Records of Hakka customs in Taiwan describe the same: sons, daughters-in-law and unmarried daughters wear coarse hemp with the collar left unhemmed and a hemp belt at the waist, while married daughters wear white hemp.
Grandchildren wear blue. One generation further away, there is no need to dress as heavily. Great-grandchildren wear green, a lighter colour again. Great-great-grandchildren wear red.
So if you see someone in red at a funeral, it means the deceased lived to see a fifth generation. Five generations under one roof is what the elders call a “happy funeral” (喜丧). The deceased left with blessings, so the youngest generation may wear red.
The colours do vary. One Singapore funeral company describes grandchildren in green and great-grandchildren in blue, while Hakka records from Taiwan give great-grandchildren blue and great-great-grandchildren red. Red for the great-great-grandchildren is the same in all of them.
After years in this work, one thing stands out: our ancestors thought these customs through very carefully. Many rules are no longer understood today, but once you know the reason, each one makes sense.
Customs differ from place to place and between dialect groups. This is shared for reference only, with no need to be superstitious. How your family does it is best decided together with your elders.