Last year, YouTube shed its lifeless, gray in-app icons for more visually distinct line-art ones. It seems like sister app YouTube Music might not be too far from getting the same treatment — the new icons are already being tested.
The updated iconography isn't tied to a specific app version, but rather it looks to be part of a server-side test. I've got them on my account, and you can see how they compare next to the older icons below:
Old vs new.
As with YouTube, the icons (and the matching text) ditch the dull gray for a brighter white in dark mode and a pitch-black in light mode. This is true for pretty much every icon you see in the app, right from artist pages to the now playing screen.
More examples of the new line-art icons.
At this point, the question is not whether Google will roll this out to all users; it's when it will be available for everyone. Unfortunately, there's no clarity on this. The earliest report that we could find on this dates back to almost two months and even now not all users have received the new icons.