JR I must confess to plagiarizing that yamabuki reference from Kodama-san in "Kokugyo". I'll just give the entire quote. Under
Kin Showa on page 70 he writes "It was developed by adding the shine of Yamabuki to Showa Sanshoku. Because the strength of the metallic shine and the Sumi thickness are inversely proportional just like Yamato Nishiki, stronger shine weakens the Sumi. Again, Koi with both strong shine and thick Sumi are considered excellent. It is a challenge for breeders to create this koi." He also includes probably the best photo I have seen of a Kin Showa with yellow, white, black, and pinky/orange color.
While I don't believe Kodama-san can walk on water, his book does happen to be the best book on koi development I currently have in my collection. The book is likely to have mistakes, some of which could happen in the translation, but none the less it is a good source of information for discussion. I like it.
Mitch