*sigh* lotta idiots and wanna be pimps in this thread... stocks are 4300 k FOR A REASON. get as close to that as you can
^^^
4300K puts out the most Lumens, and if you're running a projector setup you'll get that blue/purplish cutoff without sacrificing light output.
If you're running just normal D2R HID capsules (they arnt called bulbs) dont go above 6500K. 4300K= Sunlight, 5000K= blueish, 6000K= purplish. Just note that the higher you go up the Kalvin chart, the less lumens you'll be putting out. Hence defeating the purpose of getting HIDs to begin with.
The best color/light output setup will be 4300K D2s capsules in a projector setup.
//content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif Dont run D2s capsules in a NON-projector setup, they dont have a shield, and you'll be blinding everyone. Once everything is done, get them aimed, dont be a douche, and visit HID forums for more precise info.
Real HIDs dont go much above 6000K, stick to that # (or under) and you'll be alright. If you're running 8000K-10000K, well it might look cool, but you're basically running a strong LED and your lumens output has gone to ****.