Laptop boots to black display, then white once OS loads.

Got a tough one here guys. I'm working on a Late 2011 MacBook Pro that's exhibiting some odd behavior. Pressing the power button, I hear the standard boot chime, but instead of getting the white/grey startup screen, the screen stays black. However, once the OS has loaded, the screen comes on but stays white. If I hook up an external monitor, I get a regular desktop and I'm able to login and work as normal. On restart with the ext monitor connected, I can see the Apple logo and progress bar (on the ext monitor of course; the laptop display stays black during boot and only comes on once the OS has loaded.), and for all intents and purposes I can find no other issues with the laptop. I have tried replacing the RAM, hard drive cable, and hard drive all with working components from my own personal 2011 MBP, so I know the replacement components are good, but the problem persists. Any ideas?