Not even unaffiliated...have seen teh red ones...lol and they even do them in baby blue and pink now....I could get quite carried away. But if a total no no I think I will go for brown...
Most unaffiliated dressage is run under British Dressage rules. BD rules are that boots must be black or brown and may be top boots or Jodphur boots. Gaiters may be worn providing they are of identical leather to the boots and have no decoration of any sort.
I guess therefore that rules out even black tretorn boots.