AJ,
The difference between 4500K and 7500K is just the colour of the lighht as it is measured in KELVIN(K).
4300K is the nearest natural sunlight, and will get the most light on the road, blue look start around 6000k, with 12000K being almost completely purple.
The bulbs are a gimmick, the reason why you can't get HID look from a normal HALOGEN bulb is because HALOGEN light gives off the whole light spectrum, which the filters(colouring) on the bulbs hope to stop just letting the HID looking light through, but they are not very sucesful at it.
Another reason is because HID is an arc producing the light, and not a glowing filament that continually deposits.
The HID in the pics looks around 6000K, 4300K is so muc more effective.
tabetha