The item categories/colors are:
<li>Poor: Gray
<li>Common: Yellow
<li>Uncommon: Green
<li>Rare: Blue
<li>Epic: Purple
<li>Legendary: Undecided
<li>Artifact: Undecided

Epic items do currently exist in the game and a few have been found. Saw one up for auction just the other day! Keep in mind that Epic items are very rare.

Legendary and Artifact items are not yet in the game but will be added as soon as possible.

~Kat :)