super cute idea! but the fact that the color unlocks aren't retroactive is a bit of a bummer. there are achievements for owning the various pet colors, so it's not like it's unverifiable which pet colors a user has owned in the past. could the unlocks be tied to those?
also perhaps just my opinion/matter of taste but i think some of the colors could do with some adjustment:
Absolutely agree on both points - these could be tied to existing achievements & the colors mostly don't match the pet colors they've been labeled with. And that's not to say they're bad colors, but the hues for pet colors are pretty specific & consistent so I don't think it makes a lot of sense to label any old green as Glade, and so on. Plus, there is very much a precedent on the site for not doing that, as there's plenty of items that also bear these pet color names & they match the actual pets.
I did post a quick little comparison of some of the colors when these first came out, but what the heck, I'll just go through them all haha.
Direct Color Comparisons
The true colors are picked directly from the pet image shown. Some pet colors are more uniform than others, but I tried to pick pets that seemed exemplary of the overall palette. I did also change the text color on a few of these, just as a more minor way to include an accent color from the pet.
Angelic
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 25%, 50%, 75%, 100%[/tt]
Aqua*
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Arid
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Bloodred*
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Common
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
(Obviously common has no shared color palette, but what better pet to pick colors from than Subeta's own unofficial mascot, the kumos?)
Cream
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Darkmatter
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Dawn
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100% / font color: [/tt]
Dusk
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Field
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Galactic
Current: [tt] 0%, 45%, 100%[/tt]
True Colors: [tt] 0%, 45%, 100% / font color: & [/tt]
Glacier
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Glade
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Graveyard*
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 45%, 100%[/tt]
Hydrus
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100% / font color: & [/tt]
Lilac*
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Nightmare
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Nostalgic*
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Reborn
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100% / font color: & [/tt]
Spectrum
Current: [tt] 0%, 20%, 50%, 75%, 100%[/tt]
True Colors: [tt] 16%, 32%, 48%, 64%, 80%, 100%[/tt]
Storm
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, / font colors: & [/tt]
Sun
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100%[/tt]
Sweetheart
Current: [tt] 0%, 50%, 100%[/tt]
True Colors: [tt] 0%, 50%, 100% / font color: [/tt]
Twilight
Current: [tt] 0%, 55%, 100%[/tt]
True Colors: [tt] 0%, 55%, 100%[/tt]
omg yes your edits are GORGEOUS. i was gonna go through and make my own but i'm so swamped with schoolwork... and anyway you did them better than i probably would've lol :P
Wow those edits are great! i totally support this <3 yes, please
i need the Aqua version for sure :D
| [flower=Damon] | [tot=Damon]
edits are Amazing!! ✨ I TOTALLY get the spectrum edit. Idk why the original was made so dark.
Those look STUNNING. I just want to throw in that when these backgrounds are added, there's at least a simple webaim contrast check on the styling of the card. https://webaim.org/resources/contrastchecker/ A slightly lighter text background usually goes a long way.
creative limbo
🌷[flower=Lyonid]I have four unlocked.
I should have twenty-three by my pets achievement page. This is in there with the rest of the mess that is the achievements currently, I'm assuming?

Another (minor) bit of feedback - the cosmetics pages says this in the info on the backgrounds: "Once a background is unlocked it's yours for life, even if you trade or recolor the pet later." Since trading pets is against the site rules, I think it should say "abandon" (or "rehome" or whatever) instead of "trade."
(Also, I agree with Frenchi's points, and I LOVE Syfyri's mockups! Gorgeous 😊)
I agree with all the posts above, the color being retroactive would be a most appreciated thing. I'm not a pet oriented user so I'm not really looking forward to turn a pet in all the colors again to get all the background options. The edits look great as well
Ditto on everything!
I had no idea the user hover backgrounds I was seeing were tied to pet colors! I love that.
Although, I also have to say I prefer Syfyri's mock-ups. It bothers me that some of the color choices don't match the pets very well when the pet color schemes are usually very consistent.
+1 to the request for it to be retroactive/tied to the pet color-related achievements. Like, I guess I can just buy some elixirs and change a pet around a bit to unlock the rest of them, but it seems kind of annoying when I've already done that in the past to get the achievements.
I'm also kind of laughing because less than an hour ago I made one of my pets into a field telenine temporarily when changing species, but I didn't know about this, so he's already darkmatter and I guess I missed my chance to unlock that background without recoloring a pet again lol. Oh well.

[flower=lea]
's edits are lovely, my main gripe is with how many are so similar to each other. Most of the lighter blues are within a narrow range, but at least they're somewhat distinct at a glance.
Arid, Common, and Nostalgic are very nearly identical, same with Angelic and Cream, which makes them all incredibly easy to mistake for one another if you're not comparing them directly. If this is intentional to give users a specific color range at a potentially cheaper option (Arid vs Nostalgic, Cream vs Angelic) then I suppose it's whatever, but they probably could've picked a non-beige/tan and non-blue for Common (yes, the common potion is blue and the unofficial common kumos mascot is blue but we have six in the blue-dominant range, seven if you're generous and count the blue-grey of storm).
Also can't quite figure out why Nightmare is purple, given how monochrome/greyscale the actual pet color is. Agreed that purple makes more sense for Darkmatter and/or Storm.