This is one of my very first Arduino codes I made almost 3 years a go.
The idea is to connect a LED strip (Red, Green, and Blue) through power transistors to 12 VDC.
The LED strip is common anode, so the common + is connected to 12 VDC source while the cathodes are connected separately to the collectors of power BJT transistors.
Each transsitor is connected from the base to a 470 ohm transistor (1k ohm is fine as well). So whenever the signal raeches the base the transistor works as amplifier for the signal. that causes the RGBs to fade in and out.
Also you can do it with one RGB LED as in the link: http://fritzing.org/projects/fading-rgb-led