yeah that makes sense. I guess if I removed the AA on the tombstone tiles a bit the issue would be fixed (assuming that color 0 can be changed), because then the tombstones would have 3 colors on the tiles above/below them. 

Nice and simple, I like it. Very clean and readable.

There is one "problem" though, which would only matter if you wanna be true to spec.
What you have done is totally possible on the real GB, but if the player would be able to walk behind the tombstones the sprite would be partly visible through them.

Sprites priority can be in front of behind tiles and colour 0 is the colour that will be still seethrough, you can not make a tile totally opaque and sprites go behind it. I am not sure if which colour is 0 can be changed or if it is always the same colour, but in tiles that have all 4 colours you will always have this issue.

Same goes for walking in front of tiles, if you use the transparent colour of the sprites as an actual colour to draw. This can be seen in quite a few games where you go in front of some bg and parts of the bg will show through the lightest parts of the sprite.