Purity determines the % of drain on a powersource, if one is equipped. Think of it as conductivity; the higher the rate of conductivity, the less power is loss to entropy (eg, like electricity through copper wiring).
The base drain for any INFUSIBLE item is 4 / second.
At "Purity: 0", an item will drain 4 power every second. At "Purity: 25", an item will drain 3 power every second. At "Purity: 50", an item will drain 2 power every second. At "Purity: 75", an item will drain 1 power every second.
I've yet to see an item with "Purity: 100"; if they even exist, there's two possibilities.
1) It'll drain 1/second, because ANY infusible item will draw at LEAST that much per second. 2) It'll drain 0/second, indicating that the item is a perfect example of peerless craftsmanship, and makes NO drain on the powersource at all. (Though you must still have one to affect the stats.)
**Note that for powersources, you divide the stats of that PS by the 'purity' of the armor piece . example: a powesource that gives +150 mana would give a piece of armor with 50 purity +75 mana . (150 * 0.50=75)
The main difference between the misty & transluscent powersources is that the transluscent sources have stronger procs. For a table on the stats of Powersources (at 50% purity) check this page out: http://www.eqwiki.net/wiki/Combine_Power_Sources