It was probably the color band of the HID projectors. When you go from stock halogen reflectors to HID projectors you will notice a color band of blue, and purple. If you ever are driving at night, and a car that has OEM HIDs, ever notice it looks real blue, and once you get closer it turns to white? That sounds like what that is.
I've seen a brand new Bently last nigght, and it done the same. Just the color flicker of the HID projector. Some projectors have more color flicker than others.
Custom cutoff shield.
OEM color flicker.