This instructable (my first no less) shows how to control an RGB led using processing and arduino.
The key feature being the colour is selected by clicking on an image, the RGB value of the pixel you click on is processed and sent to the audrino which then outputs that same value.
Obviously the match isn't perfect especially at darker colours as I'm yet to see any LED do black very well, but the project should hopefully provide some insight to those of you starting with arduino and other microcontrollers.
Feel free to ask any questions in the comments and I'll answer them as best i can.
Also as i am new to instructable publishing let me know any improvements i could make on this or future instructables.
Video below shows the final result: (if it wont play right click and go to watch in youtube)