Hi,
Yeah just switch the three 'cathodes' to ground to turn the respective LED on.
Good luck getting the colors to mix well though. All three dies are in different physical positions in one plane so the colors radiate out at slightly different angles. You'll need a good diffuser to get them to mix into one single color, and you'll still see color fringing on the edges. Also, good luck getting some colors to show up good. I got a good looking aqua color, in fact very good, while a good orange was almost impossible. Could be the LED i used but i never looked into it that much.
For a diffuser i've used foggy looking clear plastic, layers of tissue paper, heat moldable plastic, anything that is colored white and is partly transparent.
So by now you are getting the picture: It is partly a current distribution problem (vary average current to the three LEDs) and partly a diffusion problem (diffuse the light from the three LEDs so they appear to be producing only one mixed color).