Your bulb could of gone bad, the ballast could be going bad, or the fuse could be going bad in your car. This is because when power decreases, the color/brightness changes and gets darker.

Easiest way to check it is to first remove and swop the bulbs. If the problem switched with the bulb...it is the bulb. If it does not change sides, then remove and swop the ballasts. If the problem switched with the ballast...it is the ballast. If it does not change sides...then it is not the HID kit. You then want to change your fuse for that side of the car to a new fuse with 15 amps.