Take a look at his thread. He didn't paint the cone. He painted the bright silver ring on the frame of the JBL driver flat black, to be nonreflective behind his screen.
BTW, nice build thread Big. I really like the final result - basic black is always in style. I've been following the threads here and on the Pi forum with great interest, and can't wait to hear your final listening impressions.
I couldn't spring for the full monte 4pis, and have opted instead for a much more "budget" LCR Ewave SR build, with a form factor similar to Brandon's No Quarter. Though I'm sure they won't sound as nice as what you've just finished up, for less than 1/3 the price, I'll be able to afford to add the second sub I've been wanting.
Now I need to start a build thread of my own...