I work with a guy that does it for a side job. He uses only Whelan products and his installs are awesome. In most States clear and yellow are legal, everything else needs a permit. They also make a unit for the rear end of cars called a backflash unit. It alternates brake and backup lights in a flash pattern. If I can find the link I'll post it.
Email me if you have any other questions:
inthedetails@verizon.net