Otherwise your terminal may be missing true-color terminal mode or the mpv in your distro doesn't have that output plugin. (You can check with "mpv --vo=help")
You need to edit spongebob-cli to add the arguments to the code where it calls mpv. It sounds like you're just running it and providing the arguments to its prompt.
https://github.com/trakBan/spongebob-cli/blob/main/spongebob...
edit: yep, works. You might also want to add "--quiet" to get rid of artifacts.