I guess the first step would be to write the Processing code?
Yes look at the Video capture from the examples menu.
It is very easy just to get one pixel from a frame.
Next you have to communicate this to the arduino. I did this in my mini monome project:-http://www.thebox.myzen.co.uk/Hardware/Mini_Monome.html
It also has the code for receiving the data. look at the lights test code rather than the monome code itself.
The problem you will then have is calibrating the colour of what you send down to the colour the lights produce, not as easy as it sounds.