Ep.19: Ellie's mission, strongly encouraged (pushed into it) by Alice, is to befriend Luke and start moving him towards their side. She has already seen how he protected her when caught distributing leaflets, and how he saved Trudy from Execution. And most recently Luke intervened when Ryan got mad with a couple Chosen guards.
Ellie takes Luke aside to ask why he hangs out with the Guardian who is obviously worlds apart from him on humanitarian issues. She pushes him a little too far and he becomes angry.
Now notice her face paint, or the lack of it on her forehead.
Ellie immediately goes to Alice for a sympathetic ear to how her talk with Luke went awry.
And on the way a new design has appeared on her forehead.