Kali: The arm band is purple and it represents her marriage to Ghira as incorporating colors on clothes/person in some way is the most common way people express their bonds to others on Remnant.
Ghira: He wears a black band on his right arm which likely his marriage to Kali
Taiyang: He wears a red bandana on his left arm
Yang:
Yang has always had a purple accessory on her outfit since V1, to represent Blake.