Advice on highlighting actors on minimap

For my top-down game a created a minimap using a second camera + render target texture. Unfortunately it doesn’t look very well because the the actors are barely visible because they are kind of with similar colours to the environment.
How could I create some sort of highlight on the actors when shown from the minimap camera? Any kind of contrast effect/trick or anything that will improve the visibility of the pawns on the minimap?

Thank you.