But to simply set the colour (Hue) can be a little tricky because you need to convert that to individual brightness value for each of the Red, Green and Blue LEDs.
Wire the circuit as shown below:
Red LED Anode : D3
Green LED Anode : D5
Blue LED Anode : D6
Common Cathode -> 220 Ohm resistor -> Gnd
NOTE: Whist a common Cathode resistor gives you the simplest wiring, the colour matching to the Hue value can be a little inaccurate because the forward Voltage drop of the coloured LEDs is very different. For more accurate colour rendition use 3 220Ohm resistors from the Arduino pins to the Anodes and connect the Cathode directly to Gnd.
Step 1: Hue and Brightness
See the picture to see the various colours (source: http://en.wikipedia.org/wiki/File:HueScale.svg)
Brightness is controlled separately.
We need some code to convert Hue and Brightness to RGB levels to go to the LEDs.
Note: The max value an 8bit number can hold is 255. So the Hue values are scaled to go from 0 to 255 rather than 0 to 360.