Hello eborr, thanks for the response.
I am afraid that your first proposal will not work for me. I am not using entities (coloured yellow in Scene editor), I am using region entities (coloured green in Scene editor), which are entities without a sprite. The drawing of the bookcase is part of the background and I simply traced the region entity around it. Therefore, the decoration region that I add does not have an effect on the region entity. It only affects entities with a sprite. Not a bad idea when using entities though
Your second idea would require to cut graphics from the background to create sprites for entities. Instead of going this way, I would follow your first idea instead and have normal sprites for entities and a decoration region to apply a graying effect on the entity's sprite.
I will wait though just in case someone has a solution for region entities, so I don't have to convert all region entities of every scene into normal entities.