If none of the monitor buttons work, it sounds like a circuit board went bad, plus the screen issue also points to a possible circuit board issue, if you went through the usual suspects (video cords / power supply cable).
Have you tried powering on while holding down the power button for like 10-20 seconds? I recall some dell screens would factory reset if you did that.
If all else fails, you know what you have to do right?
Drill more holes in it and see if that resolves the issue, its the final option.
What power button are your referring to? A computers?
Note I get this rainbow screen just plugging the monitor’s power cord into an electrical outlet (not even connected to any PC). Connecting it to my two PCs doesn’t seem to make any difference. Nether does switching any of the monitor cables. (Power, DVI, VGA, etc).