This is a RGB+White LED floodlight I'm building.. 3 watts each of color, one watt LED's. When complete, it will have two heads, for a total of 24 watts.
You are going to want to do PWM fading, which is what allows for color mixing. In this video I just have tossed a plastic cup over it as a diffuser, and I am running at reduced current so that I don't fry a retina. Three watts of LED light from a few inches away is BLINDING and leaves afterimages for quite a while.
A decent switching transistor is what is needed, I've always been a big fan of TIP120, as it can handle a lot of current and has a very high gain (meaning you can control a lot of power with a little bit going in). If I remember correctly, TIP120 can handle 5 amps, which is more than you need.
For my build, I am using smaller transistors (rated for 600mA) as I am only driving 350mA, and LM317 constant-current drivers. Power comes from the switching 12v power supply, it can pitch out 12amps if required, a bit of overkill but it was in the bins. The board I made does each channel independently, so it has eight individual power supplies and switching transistors. On the board, the power regulators are the devices with the heatsink tabs and are in the rear of the board, the switching transistors are near the front and are actually a bit smaller and have no heatsinking tab.
On the Arduino side, it's just four lines (red,green,blue,white) and power/ground. I've just got it sending random levels in this demo, I'll have a few fades and washes and the like written up in a few days.