On item tiers, the patch notes for 24 June 2013 say:
Uncommon (green): one extra power, chosen as appropriate for the difficulty of the monster Rare (blue): two or three extra powers, chosen as appropriate for the difficulty of the monster Epic (Purple): two to four extra powers, with at least one power significantly more potent than lower tiers Legendary (Yellow): three to five extra powers, with at least one power significantly more potent than lower tiers
Not sure if this is still valid, will check next time I get the chance