Blue halogen headlights aren't actually blue but are a bright white that gives off a blueish hue. Most people see these as blue because we are used to seeing a yellow tint to headlights. These bulbs are legal because they are, in fact, white. The only color headlight that is approved to use on the road is white. If you are thinking of using any other color, you could find yourself on the side of the road, getting a ticket.