Commercially available decorative lights are legal because they are always on or off. They are not blinking with music.
I think my brake lights blink with the music because my foot also does.
A "colour organ" circuit blinks some LEDs with low frequencies, blinks some LEDs with midrange frequencies and blinks some LEDs with high frequencies. The LEDs dim or do not light when the volume is turned down on the stereo unless the colour organ circuit is fed from a line level signal that does not change its level with the volume control.