I see this both on macOS (13.1, Air M2) and on Android 11 phone. Not sure if this is new, I never noticed it before. But the grey seems different for classical search results and not Pop/Rock, at least when I tried now, and I very rarely search classical
The changed happened years ago actually. And it was to quickly indicate Classical genre music. If you care, try adding Classical genre to a non-classical album and you will see it change.