you should maybe read up about casts and how they work. 
“Cause casting doesn’t check if it’s actually the blue char overlapping the buff.”
<- yes, it does exactly that, if the blue char is a seperate blueprint.
if the overlapped char is blue and you cast for blue char bp, he will succeed and you can execute his function. if it fails you make a second cast that is plugged in to the first casts “failed” path. and now you cast for the red player. if it succeeds you can execute the function. if not, cast for green and so on.
youre basically asking this: i got an overlapp, is this overlap a bluechar bp ? if yes then use exactly this char reference and execute his “give buff” function. if not, you ask for the other sorts of bps