Interface Processing with Arduino to operate a servo by MouseX
When you try and solve a problem you want to focus on just one part at a time.  If I have understood what you are doing correctly then you are trying to:


Get a computer to talk to an Arduino over serial
Get an Arduino to position a servo
Get an Arduino to decode information sent by the computer 
Get the computer to code the information
Get the computer to draw graphics.


You can try and ice a cake, feed a baby, talk on you mobile and drive a truck all at the same time, it doesn't mean its a good idea.  When you crash what was the cause?  You haven't got a clue, you were too busy trying to do all that not analyse what is happening at the same time.

So, what I'm trying to say is break it down.  Normally with motors the problem is related to power, people assume that the Arduino can provide enough power for a servo, that's not always the case.  

I would:


Write a program that sets the servo to 10 hard coded positions.
Write a program that sets the servo to the position manually entered on the command line.
Write a program that draws the gradient, interprets the user entry and writes the command to be sent to the Arduino to the command line.
Write a program that runs on the Arduino and echoes what it receives over the serial line to some "other output mechanism"  (Connecting the computer via SoftwareSerial might be a good idea).


And once they are all working as you expect connect them all together and it will all magically work :)

(Sorry if I'm teaching you to suck eggs, if you have already done all this ignore me)