Hmm....

[Pwikachu213]

any reason why shiny galarian zapdos , articuno , and moltres look just like their kantonian counterparts

 

[Alpine64]

It's a cool nod ig

 

[colours]

thinking similarly to the above poster. feels more like a nod to their kantonian counterparts, which i think is fine cause imo they do interesting enough look better having their kantonion versions' colours (especially galarian articuno)

 

[Gengarzilla]

Same reason why Shiny Zacian and Zamazenta have similar colours to Dialga and Palkia, it's so we notice this and hopefully think it's cool.

 

[Okido]

yup, it's just a reference to said Kantonian counterparts.
While I'm not really a fan of the Galarian birds in general, I'll admit it's neat they took that route for their shinies, when they very well could have just slapped some random colors on them that didn't represent anything. (and yeah, I agree that Shiny G-Articuno looks pretty sweet LOL)

Same reason why Shiny Zacian and Zamazenta have similar colours to Dialga and Palkia, it's so we notice this and hopefully think it's cool.

I was under the impression Zacian/Zamazenta's shiny colors were chosen because of their names. Zacian comes from "the cyan", and Zamazenta comes from "the magenta" - lo and behold, their shiny forms are entirely cyan and magenta.

It is interesting to think of that as a parallel to Dialga/Palkia though; I do wonder now if that was at least a little bit intentional.