Russ,
Even Blackwater Creek is calling a yellow Shusui -------Ki Shusui
Blackwater Creek: Asagi & Shusui
Sub classification is determined by the pattern of the red and blue on the body of the koi. Hi Shusui fashions hi that extends up over the back, so that the two contrasting colors are the red of the hi and dark blue of the mirror scales. These are unsubtle and quite striking koi. Hana Shusui (Hana meaning flower in Japanese) also have more red than normal, but here it is in the form of an extra band between the lateral line and dorsal fin, with a break in between. In the best examples, the hi is laid out in a wavy pattern to give a flowery effect.
In Ki Shusui, yellow replaces the red.
Although theirs is a butterfly.

If you do a Google search the term Ki Shusui is used quite a bit, although I wasn't aware of it until today.
Mitch