purple items are items that have a Mythic affix. Mythic affix are made using a combination of 4 myth stones placed in sockets on an item. the Mythic section of your Codex shows what recipe you have found from using myth stones. there is a list of all recipe’s in the forums somewhere, if you do not want to wait to find the recipe’s in the Codex.
hope this helps. also, the dictionary in the Codex has a section about Mythics, and what they do.