I found a couple of bugs in my code (using MOV instead of MOVS and wiping out the pixel counter), and I've determined my 240 pixels will probably stretch into the overscan. But even with those bugs fixed I still didn't get anything more than a B&W picture. Hrmm... But, curiously, my video capture card picks up colors although it's not stable.

Found a typo / miscalculation in the number of pixel clocks for the blanking. Getting closer, but still not 100% stable.