The Prius fog lights have a metal shield over the bulb, and a well engineered reflector, which prevents excessive glare you often get with HID conversion kits.

 

I have the factory HID headlights, and wanted the fogs to match them more closely. With the factory fogs, they looked very dull and yellow when compared to the headlights. Now they match nearly perfectly.

 

I do not recommend you do this conversion to your headlights if you have the stock halogens. You are going to get a poor result, and will have too much glare. If you want better light get some Sylvania Silverstar bulbs. They are great, and much cheaper than HIDs. Fog lights inherently have a sharp cutoff designed into them, and this allowed me to do the conversion without blinding people. I drive with them on all the time, and it's been almost one year now and not a single flash for the lights being too bright.

 

As far as HID color goes, try and stay away from the kits advertised as 8000k, 10000k and so on. They are a waste and actually put out less light than the 4300k kits. The factory lights are around 4300k, and are the friendliest to the eyes. Those other kits just look purple. If that is what you want, just get some purple halogen bubs for 1/10th the price.