When YouTube Music Key beta started, it made sort of a collage of album covers top banner image for each topic page (Topic pages are where you search an artist, and there is an auto-generated page about that artist, not owned by the artist or label, but created by YouTube automatically).
Then YouTube changed it to a solid red banner for several months.
Today it’s a banner with a subtle new pattern of colors.
See example at https://www.youtube.com/channel/UCOTIEnrx2Z71CA_U0L5i3qA
This indicates that YouTube is moving YouTube Music Key beta into it’s next phase. The service is expected to move out of beta into full launch in September 2015.