Technically speaking, federal law says that if a vehicle does not come with an HID system from the factory, it is illegal. With that said, they have a tough time proving that in court, so they will go after you with color. Once you get into the range of blues, purples, etc, you become a target. Police are also able to claim that forward-facing blue lights count as impersonation. Another member on this forum got hit with that for having blue glow emblems in his headrest nooks.
I'd say to go with 5,000-6,000K, as that will net you from bright white, to a hint of blue with your lights. Here's an example of mine with a 5K kit from DDM Tuning:
Also, Xero is correct. Temperature does not determine brightness; however, it's been shown that the further away you are from "natural sunlight", the more your visibility suffers. Hope this helps.