>>46928047This, I'm assuming a lot of it had to do with palette limitations. Like how a lot of people think Nidoqueen's shiny should've matched NidoranF and Nidorina, but it actually does. Look at their shiny palettes in pic related and you'll see they're all a near identical set of white - pink - green - black, it just turned out the way it did as a result of how Nidoqueen's original sprite was made.
You can see a lot of this blue/purple -> green shiny transition in tons of Gen I and II shinies:
>Rattata>Ekans line>NidoranF line>Jigglypuff and Clefairy lines >Zubat line>Tentacool line>Grimer line>Mewtwo>Totodile>Chinchou line>Marill>Espeon>Skarmory 