Apple Photos

OK - that one is yellow on mine also. Never noticed it being red (or even there) before. Probably changed in a recent update, and I guess it is just cosmetic. Possibly because people associate red with warning.

I also have white hearts. Same version. Can you post a screenshot?

With Photos 6.0 on Big Sur the Favorite icon in that view is yellow.

Which version of Photos are you using? Currently, in Photos 6 on macOS 11 Big Sur the favourite heart is always white, neither red nor yellow, as far as I can tell.

OK - mine are all yellow. Sorry I have no further ideas.

Could they be photos of some "bleeding heart" liberals? 🤣''

It might be tied into the System/General preference pane and the highlight and accent colors chosen. Other than that I have no idea.