Is this a liquid spilled MacBook ?

Try to light the display back cover apple icon with a flashlight and check if you see the desktop. If so, the problem could be the inverter cable, the inverter board, or a problem with the logic board (liquid spilled damaged logic board). Your backlight lamp seems to be good since the screen flashed grey at boot up.

If you don't see a faint image of the desktop then the problem is harder to diagnostic since you don't have any video and lighting. If the MacBook still display video on the external monitor your GPU (graphic chip) on the logic board should be OK. Double check your inverter and LVDS connection to the main board. Did you replace only the LCD panel or the whole display assy ? If you only replaced the LCD panel then you're using the same cables that could be suspects. The replacement display you're using could also be faulty if it's not a known working one ??? To troubleshoot this kind of problem the tech needs to use known working parts.