The music apps differ between 8.4 beta and 9 beta because the version of the music app is newer in the 8.4 (dev 4, public 3)
Personally I prefer the newer 8.4 version with album view taking on the art colour. To me it's WAY more pleasing to the eye and in many cases shows how a system wide dark mode would work so well. (All white all the time is A: boring and B: annoying on the eyes.) I think it's very slick to have a different colour that matches every album you're viewing.
Exit* Yes everyone should have at least a few of their artists with an artist image before selecting a specific album. (If they have an image in iTunes you will also see it in the music app once sync'd to the phone.)