Just like jfrolang said, 4100k's are the brightest HID setup, which will offer the best visual performance.
My friend has 3000k HID's on his EJ and they are Uber-yellow! They aren't very bright IMO. Don't get me wrong, the HID's always are going to brighter that a 5-year old OEM bulb, but when your driving down a dark road, all those colors really don't help you see any better than OEM Halogen bulbs...
Oh yea I kno. Cuz I got some POS yellow bulbs in mine and they dont do ****.