Weird, I wouldn't have thought it would be a 150m difference based on light/dark colour.
Given you would be pointing it at the numberplate, why would the car colour change your distance?
I would have thought car shape, cleanliness, amount of chrome etc would play more of a role in reflection than if you have a white or black car.
As for the law thing, I know how radar jammers are illegal, but there is nothing in the telecommunications act for lasers (
afaik). If there was, you'd have to have a license to operate a torch. I know you will get your ass reamed for having them, just trying to work out under which law to give factual backup.