I believe Agenda 18 supports searching by the colour of notes.
How?!
iOS
Side note: I’ve spent ages trying to find latest release notes or instructions in the community for this without success. Release notes appear to have disappeared, can’t find instructions. Searching on ‘color’ and ’search’ doesn’t find anything relevant except a report about a bug with searching on color in the beta for A18. Am I doing something wrong?
You can do that now, actually.
Make an overview of notes with a certain colour.
Save it.
Now, within this overview, search for notes that are OtA.
Save that overview and remove the former.
Done.