this is a small bug i encounter quite frequent these days:
when a npc or player changes hosility setting (a npc - u talk to him for some quest and than he becomes agresive / a player - in some instances now mobs and bosses Mindcontrol some of the players)
the target bar does not update color for hostility.
there is a setting to have hositility displayed in the health bar as color. dunno if most ppl use it. i have it on and hostile units have the heatlh bar completly red, friendly green, neutral (in shatrath) blue
also, this bug seems to act mostly when going from friendly to hostile. the reverse scenarios are much more rare though to test :)