All the guards in mlp are waaay too clone-like for it to just be coincidence. I know in later seasons they added a few more colors to the guards than just white-blue and grey-white, but there’s still too many identical ponies running around for my taste. So I decided to headcanon that away.
 
 
There’s a reason human soldiers wear the same clothes, walk in step with each other and such; it looks a lot more heckin intimidating to have a large square group of identical people marching towards you, than if they were just a blob of people walking towars you. Now, for humans, we don’t vary that much in appearence when wearing normal clothes. The only visible thing is our faces, and those are difficult to see from a distance. So for us it works to just put us in identical clothing and call it a day.
 
Ponies, on the other hand, are a mess of rainbows that don’t really do clothing. Sure, full body armor does exist, but who wants to parade their pretty soldiers around the town when they’re wearing giant tin-cans? That’s a recipe for a scared public and fainted soldiers.
 
So, ponies ( i.e. unicorns, who were the most concerned with appearence at the time of pre-equestria) invented spells that would alter the colors of a pony. Originally cast onto each soldier every morining and having to be reapplied every few hours, until enchanting objects became a thing and the armor itself could sustain the spell provided magic recharge every couple of weeks. Now, this spell doesn’t actually alter the appearence of the pony; it creates a color-illusion which is placed on their coat and mane, causing everything seen through it to appear as that color. This illusion was originally very crude, and a stray wind ruffling someones fur would cause it to ripple in the underlying color; manes were a big no-no, so they were kept wrapped under the helmet and armor, with faux manes and tails glued to the outside. Nowadays, the enchantment is much more sophisticated, with each strand of fur being colored separately. Which means soldiers can now have their real mane blowing dramatically in the wind with no problem.
 
Which colors they are changed to varied over time. In Unicornia, they usually just picked which colors looked best together with the armor. In modern equestria, it’s now tradition to appear the same color as the captain of your division. That’s why Shining Armor appeared as his own colors; he was the captain, so everyone under him had his colors. (I’m gonna ignore how they were also white-blue when he was a kid…). It’s also common for the guards to change color when ordered to, such as for camouflage.
 
In equestria, as previosly mentioned, the armor itself holds the enchantment. Since metal isn’t the best “battery” for storing magic energy needed to run the enchantment, the star-shaped gem badge also acts as a magic storage, as well as an ID. I’m gonna ignore the show on this one, and say that you can’t just steal a badge and walk in -it’s a two step authentication; having the star is just the first step, the holder also needs to have the same magic signature as is keyed to that specific star for it to work. (This isn’t perfect though, so theoretically someone with just a similar signature, or someone who can alter theirs, could also use it.
 
The crystal empire doesn’t have many unicorns that could enchant armor, so they found their own solution; they are really good at writing spells into crystals, so instead they just wrote the illustion spell into a crystal the guards have to wear. Those are the small spheres seen on the front of the guards armor in the show. Since the thing casting the spell is also the thing holding the magic, it can store less magic than the equestrian counterpart, but is also much more effective, so they usually last around the same amount of time anyway. Though, it means they can’t be removed from the armor like the stars can, or the spell will stop.
 
In this image, it’s quite a bit after the show, so the armor has undergone changes. For example, the crystal now holds a lot more spells, and is therefore quite a bit larger. These include things like being keyed to a specific person (like the equestrian version) and only working when that person turns them on. And having way to many protections spells on them making them very hard to damage. They are also “locked” to the armor unless being removed by the pony they’re keyed to.
 
 

 
 
..or at least, they’re supposed to be. Remember how I said “theoretically someone with just a similar signature … could also use it.”? Yeah, turns out Faerie has an incredibly similar magic-signature to Luminous Spirit! (Not too shocking, family members signatures tends to resemble each other) Something which the former is thrilled about! And the latter is thrilled about.
 
It’s gonna be a long guard shift Lumi. Just hope he doesn’t realise this means he can wear the armor and turn it on to change his own colors (Something he didn’t know the last time he tried to borrow it)