I am sorry - but I am not versed in Miyata. They were sold as Miyata here - not Koga-Miyata. I can confirm the grey metallic being a rather common colour on the better Miyata: s over here. The only three colours I remember ever seeing on 80ies "Swedish" Miayta: s are metallic grey, solid black and solid bright red. Importer was Sjöstrand Bike Company in Göteborg (Gothenburg).

The only Miyata I have had is the Mixte one ten II -that I built (and hand painted flowers on upon the original black paint) for my mother many years ago. Sorry for not being able to help more.