i think it's wonderful how in other stories love interest want to monopolize their significant other and that includes their art, but here, aya wants mitsuki's music to be heard by many! the love of music they share is amazing.
I am not firing on all cylinders right now, so it feels like the color/green is symbolic of something here when its Aya thinking and seeing others and them together and so on, but I can't figure it all the way out so I also feel like maybe I'm Charlie with the red thread in the mailroom.