Changing LEDs colours as a car goes up in RPM?

i'm buiding a buggy and would like to build a visual effect when going up through the RPM and have some LEDs in the back change colour as i complete in the night events,   
My question is there  away of building a system that  runs off the aultnator as the RPM rises it changes the colours in the light out put

There are many ways of building a system like that. You'll need a micro controller to take the given input and covert it into the right color combo on your RGB LEDs. Its just a matter of finding a good input source. Reading the voltage fluctuation in the alternator may not be the best option.

The engine has an RPM gauge right? Is it a mechanical or electronic gauge? If its electronic then getting the output from that gauge and feeding into the micro controller at a safe voltage would be your best bet.
A few wraps of wire around an iginition coil/spark plug wire would give a decent reading that could be fed through a zener diode for protection then an op amp and finally into the micro to give rpm...