HERE'S GALAR!
Raboot & Cinderace: Change from white rabbits to gray rabbits. Being sport players, the navy part of their faux clothes (track suit for Raboot, sport uniform for Cinderace) turn to white possibly referencing them being on another, possibly rival, team.
Sobble, Drizzile & Inteleon: Chameleons can change the colors of their scales to camouflage with their surroundings. While their normal colors are more "natural", their Shiny colors show how drastic and vibrant they can become.
Skwovet: Based on the
Eastern Gray Squirrel (a species that have become invasive in the UK), its Shiny colors turn it red like its evolution Greedent. Greedent (and thus Shiny Skwovet) are based on the
Eurasian Red Squirrel. Eurasian Red Squirrels are native to the UK, but are now endangered as the gray squirrels are outcompeting them (which could be why the clever Skwovet is gray and the dim-witted Greedent is red). Though being Greedent's Shiny doesn't swap colors (just turning a different shade of red), I'm not sure what other connections can be made.
Rookidee: Based on a bird in the
Paridae family, such as chickadees and tits, who both come in blue (such as the Eurasian Blue Tit) and yellow (such as the Great Tit & Japanese Tit).
- Corvisquire: In addition to ravens, is also likely based on Jays who come in blue (such as the Blue Jay and Steller's Jay) and white (such as the Eurasian Jay).
- Corviknight: Shiny Corviknight's armor coat either resembles the color of normal steel armor or is meant to be a reference to a White Knight, a mythical heroic knight that can be seen as the counterpart to evil (usually) Black Knights. A stronger case can be made for the latter if you consider the red highlights (a color usually associated with evil) of Gigantamax Corviknight turns yellow/gold (a color usually associated with good).
Blipbug: Blipbug's yellow "bow tie" and "vest" turn a grayish-blue to further push the "nerd" design.
- Dottler: Dottler's yellow dome shell turn grayish-blue making them look even more like radomes.
- Orbeetle: Orbeetle's red shell & parts turn grayish-blue, white face turns blue, and yellow antenna & feet turn light red. This both references blue ladybug species and also, as Gigantamax Orbeetle, makes its UFO form look more extraterrestrial (the grayish-blue shade resembling a night sky) and menacing (the yellow domes under the saucer shell also turn light red making them look like lasers or "warning" lights).
Nickit & Thievul: These Red Foxes change into
Silver Foxes (also called "Blue Foxes"). Silver Foxes are actually Red Foxes which have an excess amount of melanin turning their red fur either gray or black (which can appear blue in certain shades).
Gossifleur: Depending on the species of cotton plant (
Gossypium), their flower can be yellow or pink.
- Eldegoss: Based on the seedpods of windflowers (Anemone), they come in a variety of colors including yellow and pink.
Wooloo & Dubwool: Change from white sheep to
black sheep. Sheep usually have white wool because its a dominant gene. A black fleece is caused by a certain recessive gene that only a few white sheep have; even if a white ram and white ewe have this gene, the lamb would need to inherit both parent's black wool recessive genes (instead of their dominant white wool gene) to have a black fleece. There's also the idiom "black sheep" but it doesn't seem to have any significance here (mentioning simply to acknowledge it).
Chewtle & Drednaw: Their skin (and shell for Chewtle) turn green, the color most commonly associated with turtles. If there's anything additional, Drednaw takes slight inspiration from early ironclad warships which were wooden boats covered in metal armor casing; their Shiny colors thus could be seen as darker colored wood (possibly from age) encased in rusty armor.
Rolycoly: Its coal body has turned from black to grayish-blue, as if the coal has been burned out. In addition it looks a bit like an engine.
- Carkol: In addition to burnt coal, since its the body & head that turned grayish-blue it looks even more like a metal mine cart (filled with coal).
- Coalossal: In addition to burnt coal, since its the entire body that turned grayish-blue it looks like a metal furnace (filled with coal).
Applin, Flapple & Appletun: Instead of using a red apple it uses a green apple.
Silicobra & Sandaconda: While also reflecting colors snakes usually are, it turning grayish-brown could be referencing "
domestic/house dust" (and maybe dust blowers). It also makes Gigantamax Sandaconda look like a tornado with dark gray clouds.
Cramorant: As mentioned above by
Kin+ak (✧∇✧), while based on a Cormorant, its Shiny is possibly based on stories of
seagulls that fell into & got covered by orange curry sauce.
Sizzlipede & Centiskorch: Based on Fire Centipedes & possibly other centipedes & millipedes which have bioluminescence, their backs turn teal resembling the color some of these bioluminescence bugs glow when in the dark.
Grapploct: In addition to many large species of octopus being red, it also looks more like a masked wrestler such as a luchador.
Galarian Zigzagoon, Linoone & Obstagoon: Taking design cues from
Glam Rock bands (notably
Gene Simmons's stage persona "The Demon" from Kiss), while the normal colors reference the bands which go for a more goth motif (such as Kiss), the shiny colors references the other end of the spectrum, bands with a "bubblegum" motif.
Perrserker (& Galarian Meowth?): Taking design cues from Vikings, Perrserker's silver/iron parts turn bronze. The
Nordic Bronze Age was an important time period for the Scandinavian people as, while it was before the
Viking Age, the culture shifted to a warrior focused one due to bronze metalwork creating stronger weapons. Since the Scandinavian people couldn't produce bronze as they lacked tin deposits, they traded
amber for bronze (and amber just so happens to be a similar color to bronze). And in order to trade, they needed to sail, becoming more seafaring folk which also would later be further emphasized during the Viking Age (which also leads back to the basis of the Galarian Meowth's family, the
Norwegian Forest Cat, which Vikings kept on their ships as pest control). In addition to all this as a possible inspiration for the Shiny colors, in Norse legends Thor gifted
Freyja two cats which are commonly named Bygul (Honey) and Trjegul (Amber), Perrserker's color scheme matching the typical colors of those two things (and thus also Galarian Meowth to a lesser extent).
Galarian Corsola & Cursola: Based on bleached coral, their Shiny darkens their color to almost black. There are species of coral called "Black Coral" (
Antipatharians), which in addition to being at risk of bleaching due to climate change, have the problem of people harvesting it (often faster than it can reproduce) and made into jewelry (Cursola looks sorta like an ornament, especially in its Shiny colors).
Sirfetch'd: Changes from a white duck to a yellow duck. This could just be referencing it being a duck (even though its mostly ducklings which are yellow). Another possibility going off its knight design is that, like with Corviknight, Sirfetch'd is meant to be a White Knight, and what color is more radiant than white but gold?
Galarian Mr. Mime & Mr. Rime: Based on a
vaudeville actor (G-Mr. Mime a general tap dancer, Mr. Rime more directly based on
Charlie Chaplin's
Tramp character), their Shiny desaturates their colors almost making them black & white monochrome, just like the early films which often portrayed vaudeville performances.
Galarian Yamask & Runerigus: Based on Viking Runestones, while I have nothing on the pigment and eye color, the stone changing to white could be a reference to limestone, a popular mineral Vikings used to draw runes on and also
collect to make into a whitewash for buildings.
Milcery: Normal is more like cream while Shiny is milk. Cream is yellow because it's made from the butterfat of milk, butterfat carries the pigment of the plants grazed on by the cattle that made the milk (milk appears white because the buttermilk is mixed with water and other proteins).
- Alcremie: Even though its normal forms are all based on flavors, that doesn't mean the Shiny is too; Shiny Black Pokemon is sort of a trope. Assuming it is based on a flavor and isn't food coloring, my guess is either cocoa (possibly cookies & cream) or burnt marshmallow. Gigantamax Alcremie cake layers may also be different flavors.
Falinks: Shiny looks to be wearing brass armor instead of gold, which would be a metal armor would actually be made of. The horns are based on crests/plumes that helmets worn by Spartan and Roman soldiers sometimes had. The color of the crest/plume didn't signify rank or allegiance, it was merely for decorative & possibly intimidation purposes, so them being a different color doesn't mean anything.
Pincurchin: Shiny makes it look more like an actual urchin.
Eiscue: The ice turning pink is strange, but could be a reference to
Watermelon Snow, snow that contains an algae called Chlamydomonas Nivalis which colors it pink.
Galar Fossils: Their Shiny either are supposed to look like aged corpses (since logically these mixed-up dinosaurs shouldn't be able to live) or maybe like statues to reference the
Crystal Palace Dinosaurs.
Gigantamax Duraludon: The Shard, the building which its based on, is covered in lights which can make the Shard any visible color (or mix of colors). Without lights the building's reflective material could appear blue, but it would need the lights in order to be red. A notable time when it was lit red was during its inauguration on July 5th, 2012.
Zacian: Goes all in on being "The Cyan".
Zamazenta: Goes all in on being "The Magenta".
Eternatus: Dynamax energy is associated with the color red (red glow filter, red clouds, Max Raid dens shoot out a red beam & inside the floor & ceiling is red) so a Shiny Eternatus is completely red for emphasis.
Galarian Slowpoke: There are some weird mind games being played with the Galarian Slowpoke family. Starting with Slowpoke, it's simple. Since normal Slowpoke is completely one color and G-Slowpoke has the new yellow forehead detail, Shiny G-Slowpoke is made completely yellow as a reference. Okay.
- Galarian Slowbro: Now you'd think they'd do something similar with G-Slowbro. But the thing is, G-Slowbro's new additional color is purple, which normal Shiny Slowbro also is (though a bit of a lighter shade). So Shiny G-Slowbro is just a darker purple normal Shiny Slowbro with a Shellder arm cannon? No. Get this: Shiny G-Slowbro's purple parts turn to the same shade of purple as normal Shiny Slowbro, and its pink body turns to the paler pink of normal Shiny SlowPOKE.

- Galarian Slowking: Finally, Shiny G-Slowking actually tries to match the changes with normal Shiny Slowking: Both Shiny's bodies are the same darker pink (disregarding Shiny G-Slowking's purple head and tail tip), The colored/darker stripes on their neck accesory has turned blue, and the gem on the Shellder crown has turned pink.

Galarian Weezing: Shiny looks to have either rusted (or possibly oxidized) or its redder tint is meant simulate what its normal color would look like under a haze of pollution.
Galarian Bird Trio: Do you really need me to tell you?...
Gigantamax Gengar: Now most Gigantamax Pokemon follow the Shiny coloring of their base form. But G-Gengar is the sole exception as, instead of a lighter shade of purple, it's white (or as much white as the red filter let's it be). This is obviously a holdover idea from Mega Gengar which Shiny is also white. Makes you wonder why they just don't make base Shiny Gengar white if its a better Shiny color for it!
Gigantamax Snorlax: Combining these two together as they do the same thing but in different ways. Most of the time when a Gigantamax form adds in some extra details, those details don't change for its Shiny form. BUT G-Snorlax and G-Garbodor are exceptions. G-Snorlax is more subtle, normal G-Snorlax looks to be in the Spring season with green leaf trees & pink flowers, but Shiny G-Snorlax is in the Autumn season with orange leaf trees and red flowers.
- Gigantamax Garbodor: For G-Garbodor, the colors of the "toys" have changed. Notably, the blue airplane turns red, and considering the markings on the plane it seems to be a reference to Latios & Latias. Though I'll also point out a major missed opportunity: not changing the Clefairy Doll to resemble a Shiny Clefairy. That should have been a chef kiss detail right there.