This super bright red, green, and blue LED module can be used to generate hundreds of colors. The module comes with MOSFET switches used to turn on each LED. Pulse-width modulating the MOSFETs can allow you to mix color ratios. Module documentation and application info are available to get you started right away. This is open source hardware, all design files are available.
User Data Sheet
PCB, schematic, and bill-of-materials
BM014_Arduino_Simple_Test.zip - Arduino Uno3 code drives the red, green, blue LEDs with PWM.
BM014_Arduino_RGB_LED_colors.zip - Arduino Uno3 code example that uses PWM signals and cycles through a variety of colors.