JR: Don't really disagree. That's why I said a "sort of" hybrid vigor. The color/skin aspects would seem to be risking reversion. The size/growth/hardiness aspects are what I was thinking about.
A few years ago, I helped my daughter do a crossing of guppies for a science project. Crossed a purple veiltail with a gold-bodied red delta from an award winning line. The brilliant red was lost in the first generation. Also, no purple. There were a few throwbacks to "common guppy" roots, but otherwise the fry grew quickly to become uniformly olive-bodied with dull, muddy red tails with nothing to be said for them, except: the tails were among the best shaped veils I've ever seen (combining the best attributes of the parent lines), grew larger than either parent strain, had excellent bone structure, hardy & adaptable to water conditions. They were the best looking valueless junk I've ever grown.
Asagi genetics are a mystery to me. Hopefully among the junk that any koi cross is going to produce there will be a few that take the best of both parents without losing the blue that makes Asagi so interesting. Glad I'm not the one with the responsibility to cull!!
Dick: If the male went into soft water with neutral pH, would the head lighten? How long? Has he done this color change before? Someday I'd like to try Asagi, but our hard, alkaline water deters me.