follows up on T261415: Watchlist Expiry: implement ability to see how many days left when in watchlist (mobile, advanced)
Not sure, what benefit you see with this change, the visual difference is basically invisible to most users in most device contexts. It's less performant, and adds another icon. We chose in OOUI deliberately to rely on opacity and you were in support of using filters instead of custom color icons in the past.
I pledge for status quo for reasons above.
Once more, now I got to read the task. The commit message wasn't fully clear for me about the 'why' – I'd still think working with opacity makes more sense, also when we might pick up things like dark mode in the future. And the opacity icon values are part of WikimediaUI Base vars.
Not fully sure why the opacity isn't applied to the icon currently without looking into the applied CSS though.