In your Animation blueprint set an value (bool) that indicates that your char is in idle-mode.
In your third Person blueprint get your Animation bluerpint (cast to) and get the bool from your idle-mode.
In “Event beginning” make a node “create dynamic material instance” from you your mesh. Your material must have a vector3 value that can Change the Color. Name the Parameter Color. In create dynamic material instance choose your material instance. Promote this to a variable. Then over Tick you can check (branch) if the idle-mode is true. If yes then set the promoted material->set paramter. You get a node in wich you type the Parameter Name you want to drive.
This is only a quick Explanation. For deeper Detail you have to look at materials.