I (mostly) figured out the numbering system, and figured out the bell level; it doesn't have any icons beside it anymore, but it's still not turning gold. Why not?
In Fez, a room does not turn gold until you do all of the tasks corresponding to the icons, as you have discovered, and you find every exit from the room. This means that in your situation, there is a door that exits from the bell level that you have not yet found.
Most likely, you have not yet lowered the water. There is a puzzle in another nearby level that will lower the water level in several levels including the bell level. When you do that, there will be another door that was previously below the water line but is now accessible.