#latias

Latias: Shiny Latias is pretty; a nice yellow-orange color with green accents makes for a pretty distinct shiny compared to the original. Interestingly, the eyes and markings actually match here, as on regular Latias the eyes are yellow while the markings are blue. I think I prefer them matching; otherwise, it feels like the tertiary colors are competing. No idea if there's any rationale behind the color choices, but still, it's nice.

Mega Latias: Regular Mega Latias changes to purple, so it holds up that the shiny would also change color, this time to a slightly yellowish green. I don't like it as much as the yellow, but it matches Latias' markings well enough. Biggest issue is that the eye gets lost, but to be fair the original shiny also kept the eye color of regular Latias so I'm not sure how you'd avoid that.

Latios: Shiny Latios goes from blue to turquoise, and its eye and markings (which unlike Latias already matched) go from red to orange. This is still a nice-looking shiny, though it's not quite as distinct as Latias. The two shinies do look nice together though, which I think is important given the context. Not bad at all.

Mega Latios: Regular mega Latias and Latios are both purple, so it stands to reason that their shiny megas would also match. The shift between the shinies isn't as extreme as Latias, but you can still tell them apart easily enough. The eye also maintains consistency with the non-shiny just like the regular mega does.