Since I mentioned the opacity trick, I figured I'd post an example of it.
How it works: 1. Create a low priority sprite (the character/object you want the silhouette effect of) 2. Create a high priority sprite of the background that you want the sprite in step 1 to walk behind 3. Draw a high priority tile that you want to "bleed through", usually a darker color (the example uses a highlight effect)
Palettes pal,1,0,16 LoadTiles Tile1,2,100 a = AddSprite(1,1) PropSprite a,1+Priority(0),0