Macbook Pro (Mid 2012) screen is just white no picture
That is a tough one. the Mac sees that there is a display there and even has it's resolution and other specs. So the OS sees and enumerates it, yet only the back light is on.

That may indicate that there is an issue with the replacement display, or that something was damaged or not installed correctly (crossed, damaged wire/conductor/connector).

You are troubleshooting hardware and there is not much else to do than go through the install procedure and verify that everything is connected properly. In other words, remove and reinstall.

Also do you know for a fact that the replacement display is both the exactly correct one and that it is, itself, working properly? It is also entirely possible that you got a defective replacement display.

I would imagine that doing it yourself means that you would either rather not send it off for repair or can't afford that option. If so it may be time to look into that latter option...