What are the differences between the yellow, blue and purple bosses and their respective drops?
"Technically, they're all considered as Elite and as per the Game Guide:
Champion monsters have their names displayed in blue. They travel in packs and are more powerful than regular monsters: they take more hits to kill, do more damage, and have abilities called affixes, which enable them to do things like teleport away from your attacks or unleash bolts of lightning when they’re struck.
Rare monsters have their names displayed in yellow. Rare monsters have more Life than normal for their type, possess a special affix, and are usually surrounded by a pack of three to four minions who are weaker but possess similar abilities.
Unique monsters have their names displayed in purple. They’re usually more powerful than champions: they have more health, do more damage, and often have multiple abilities."