The Elitist Elite enemies have a colored name, Yellow enemies are a single elite unit with a group of his henchmen that basically have his abilities and are a bit tougher then regular units.
Blue enemies are 3-4 enemies, they guys are the worst they have their own deadly abillities and can be a pain to beat.
Purple enemies are quest related, actually out of the 3 are the easiest to kill.
More info on them:
http://www.diablowiki.net/Elite