I would paint the walls Behr Day Spa Leave the Vanity as is, change out the a softer light fixture. Yes, to tile up the back wall... a sea glass tile (google Sea glass tile, there are many options) will add a bit of subtle color and shimmer. the counter top won't look so dingy, as it might with a white contrast.

Do I paint the wall before the tile is put on ? Great idea. Beautiful

I have a bisque quartz vanity top and a sage green vanity. A sage green vanity would look lovely with you darker floor.

Great idea

Only thing is the bathroom adjoined has blue penny tile

Kinda aqua blues

You could paint or replace the vanity wiht an aqua paint color to match your bath tile.