Here on e621 we already have a tag for better_version_at_source, indicating that the image on e621 is not the best version available, since there is a better version not on e621.
However, there are also some cases where e621 has multiple versions of an image posted. Consider these:
post #3410928 post #3508354 post #3223704 post #3337554 post #4055303 post #4176410 post #2099354 post #2129766
For each of these pairs, the one on the left is arguably a "worse" version than the one on the right. The left image is an incomplete monochrome version of the artwork. I understand why e621 doesn't want to delete these, because some people may want to see the incomplete art, or prefer monochrome art, etc.
I'm wondering if we should have a meta tag for this, perhaps "better_version_on_e621", which marks these images as such. This way, users can blacklist "better_version_on_e621" if they only want to see the highest quality version available of a given artwork. Note that blacklisting monochrome does not fulfill the same purpose, because I do not want to exclude images which are only monochrome and have no color version available, I only want to exclude an image from e621 searches when it's an easy call for being inferior to another image on e621.