Blues might have loot from farming. Reds will have a bag of reds and some leather armor 99% of the time.
I understand that but usually when building a character for pvp/pking it's not about the phat lewtz you might acquire like it was early live days. It's about the satisfaction of pvp and killing actual players.
re: Should I kill reds or blues? YES Honestly I think you should go for reds since they will be more of a challenge in most cases