help with lights

ok heres the deal, i want to get 2 undercar light kits, one red one blue, and install both of them normally,and have a switch for each inside the car. but heres the problem, i want a switch and/or knob that will turn them both on, but alternate between them, so it would be red, blue, red, blue..... and the knob increases the speed of the alternating, but i have no idea what piece i would need and how to do this. has anyone got the name of the part or a link to a site that has it, or any ideas on how to make it work? :confused:

try installing it with a turn signal relay in line (kind of like the way the police install their flashing headlights).

There are also computerize systems that can even be hooked up to your stereo and flash to the beat.