@UrsaManner  
yeah, it’s adobe media encoder that I think does the proper colors from RGB to YUV  
Glad I could help :)

So, after trying different (or not so much) ways i figured out one. Export from Animate, convert to h265 via Adobe media encoder, then to VP9 webm via handbrake. Probably not the least time-consuming method, but the result’s colors appear to be correct, so the problem’s apparently solved.
 
@Rainboom Dash  
Huge thanks for the info and help.

@UrsaManner  
If it’s a flash video then it exports RGB by default and it’s probably not doing something right  
What I use is apple pro res with Adobe media encoder  
It seems to export mostly proper colors and had less banding than lagarith (although, I have to assume something is doing something dumb for that to happen)  
And then when you compress as VP9 it should work pretty well  
There’s probably a better way but.. imo it works perfectly fine (and I’m super obsessed about getting the best quality at reasonable file sizes)  
At least, I assume it’s flash if it’s Adobe animate

So I tried Handbrake and with properly edited preset, VP9 and no filters colors still alter  
https://imgur.com/a/8P3L2hv
 
Adobe animate can export webm with proper colors (file>export>export video) but with absurd file size (>400 mb for this one), I wonder if it exports colors in RGB or messes things up in a different way. Though it shows me colors in HSB, RGB and Hex decimal (#672C8C for example) units in the color window.

@UrsaManner  
If it’s in RGB I can help  
YUV and what I listed should work fine

Thanks for all the info, I’ll try and see what’s best.

@Background Pony #D990  
Or you could, you know, just convert yourself with Handbrake or ffmpeg
 
If you use Handbrake make sure you set to “slower” or better.. ffmpeg do -deadline good  
And make sure you change the toxic defaults of Handbrake (like interlace detection, automatic cropping, lock to 1080p highest, 30 fps no matter the source)  
I have noticed Handbrake gets better compression with flat-color animation by default with VP9  
I use ABR.. what I do is use x265 to determine bitrate than compress with VP9  
You could try a quality setting but I think the only time I tried it is with Handbrake a while back and it was kinda broken.. dunno if it was fixed.. and I haven’t tried with ffmpeg
 
If the colors still alter after that then it’s probably in RGB and will need to be converted differently

Try convert-video-online for converting to WEBM

Good Animation

“I was planning to upload webm as well but turned out my convertor messes up colors”  
Is it using VP8? VP8 pretty much always alters colors

That look on Glimmer’s face at the end.