[DustyPorViva]

Well in that case I accomplished it via creating shadows black, on a separate layer, and making the layer 33% opaque, I believe.

Or you can go the route of just filling it with an opaque bucket fill. Using layers makes it easier to control what you're doing though without messing with other colors on the tileset. Also easier to control the transparency as once you fill it with an opaque bucket fill you have to fill it back 100% and then redo the fill if you mess up.