Was the original display working perfectly before trying to install the color DMD? If not, what symptoms did it have?

Check all your ribbon cables for and ensure you have pin 1 identified on the boards lined up to the red tracer side of each ribbon cable.

Double and triple check you have all the ribbon cable connector lined up properly and not one pin off up or down or left and right.

So when the cables are all connected, the game will not boot up into attract? You need to solve that first.

So leave all the cables off the actual display and just connect the ribbons between the stock boards. See if the game will start up into attract mode. If not, you either have a ribbon cable installed incorrectly (backwards or 1 pin off) or you blew one of the boards.

What splash screen are you seeing? Picture?

Make sure the coin door is closed when checking the power LEDs on the driver board. Are they lit with the coin door closed?

Maybe you should go back and try and get the original display working perfectly before continuing with the Color DMD. You said you tried the original display but it had issues. What issues did it have? Maybe installing the color DMD power cord backwards fried the display board...

Are you near anyone else with pinball machines that might have a game to try swapping parts?

Pin 2 and 3 are 12V unregulated but 27V is very high even considering it is unregulated. Are you using a quality meter to measure this?

Pin 1 on J16 measures -12V to ground? What is the actual voltage measured here? (if this powered by the same AC supply as the positive 12V supply. If this negative supply is normal and the positive is not points to BRDG3 being the cause)

Put your meter on AC and measure between ground and J17 pin 2 then ground and J17 pin 3. (This is the AC input to the positive and negative 12V supplies)

While you have your meter on AC, measure from J16 pins 2 or 3 to ground. (This measures the ripple voltage on the 12V unregulated supply)

Is LED 203 lit?

Since the voltages are correct, I agree, best to find someone that can trace signals to see whats going on here.

Thanks for posting the results! Congrats on getting it back up and running!