I too was going to try something esoteric but went with the pale blue in the end. All references I found seemed to indicate this was THE colour. I seem to remember seeing a white one somewhere but cannot recall where.
As they were so "comic opera" and obviously trying to emulate other world powers I really would have thought anything goes but sources do not seem to back this up? Also, as yellow was historically the reserve of the emperor I would have thought at least one of these guys would have had the ego to use it on that basis.
Not much help - sorry.
Svennn