I have always been fascinated by the variety of doudai available and have spent hours in Japanese bogu shops staring at the selection. For some reason the more outlandish models are kept on the top shelf, (rather like magazines in the newspaper shop). I have seen dou covered in deerskin, wild boar skin and even bear skin –all of these with the hair on. I can clearly see the advantage of these in shiai. If your opponent hits dou, the shinpan would not be able to hear it. However in the forty plus years that I have been doing and watching kendo, I have never seen anyone wear one.
Only slightly more common, are the dou with lacquered designs, showing scenes and animals in glorious colour. One of the few people I have seen wear one was a gentleman who regularly turned up at shiai during my time in Kansai wearing a glorious confection of gold tigers and dragons chasing around on a vermillion background. After making a startling first impression with this piece of kit, he invariably got beaten two nil, packed his dou and returned on the next occasion to display it again.
More recently there has been a trend back to the old style take-dou where the original bamboo slats are visible in their naked form. Then you have the new style car paint shop and iridescent moulded finishes and the choice goes on.
I have had two non plain black versions during my longish kendo career. One had a slightly cracked baka-urushi finish and a blue suede mune. The other was a vermillion dou, now faded to a tasteful brown, which I received as a present from my sensei. Both have mysteriously become too small for me, so I passed them on to Alan Thompson and Max Davies, both British Squad members. It was actually quite a buzz to see two medal winners at the recent Mumeishi taikai wearing my old bogu.
There really seems to be no rule about which colours can be worn at which grade and it remains a matter of personal taste and confidence as to what can be worn. I notice however at occasions like the Kyoto Taikai, the majority of Kodansha (6th 7th and 8th dans) go for a plain black dou dai. It was once explained to me by a teacher who had begun his career pre-war, that black was the senior colour as in black belt, (yes! I know there are belt colours above for kodansha). In his view black had been borrowed by folk lower down the grade scale until it became the norm.