Yeah has a cooldown of 200 keys. So the promo is every 200 keys you can get a guaranteed purple. You could still hit purple in a different key, it's just every 200th is guaranteed while the promo is going.
In other words, "Lucky Purple" is a completely USELESS and unobtainable trick, because unless you're so rich you can blow a fortune in real money on buying bunches of keys for the few days the promotion is active, you will have no hope of ever getting a "Lucky Purple"
Besides lucky purple (which takes really long) I would make use of the extrasensory perk as much as possible. That's how I got a lucky item during this promo using daily and a few earned keys.Feel free to take a look at my suggestions:
200m xp prestige,