Facing TV Problems? Ask a TV Technician.
Did you get BOTH parts from Mitsubishi, or other suppliers ?
Well, I've had problems with lamps and DLPs from other suppliers. Mitsubishi won't cover your lamp, but you may be able to get that DLP free if you pay the labor to install it. I think your model has that DLP listed as a defective batch from TI.
I'd get the lamp, reinstall that original DLP, see if you get those Dots back, then call Mitsubishi about the DLP coverage.
It's usually impossible to damage the black cable that supplies picture to your DLP, and it's unlikely you forgot the direction that thin ribbon cable mounts (upwards) in the DLP's board - You got a GRAY screen which means you lost picture, but the lamp lights ok before the lamp died.
You can usually see the lamp light up and stay on if you watch it from the back of the TV set. I usually watch it for 2 minutes after the TV turns on. If its bad it won't stay lit for that time.
For that lamp to even light, your TV's color wheel has to spin up to full speed. So if you see the lamp at least ARC, you know the color wheel is at full speed.