I set up a simple team color function with rep notify to change the color of the actor when the team index changes. How can I check the other characters team index value to apply damage?
What exactly is your issue and how is your damage system set up?
For example in a FPS you would just have your bullet and once it hits you can compare the team index from instigator with the team index of the hit actor. If they are different apply damage.
I am currently using a on overlap volume. I am not sure how to grab the overlapping characters team index value.
When overlapping you get a “other actor” variable. You simply have to cast it to the class you use for team characters and can then get this variable via a simple "Get ".
Thanks! The step I missed was dragging off the “as Character” pin to get the variable.
It’s always the small things which keep you stuck for hours 
Glad I could help!