Did you successfully add the expression to the fill color (the expression is there and enabled)? When you apply the fill it defaults to red. Are you saying that it stays red when you apply the expression?
toComp() is relative to the coordinate system established by the Camera view. A good example of when you would use toComp() instead of toWorld() is this opacity expression that makes a layer invisible when facing away from the camera:
toComp() is relative to the coordinate system established by the Camera view. A good example of when you would use toComp() instead of toWorld() is this opacity expression that makes a layer invisible when facing away from the camera: