Hello! I have a Late 2013 nMP bought in early 2014 which originally came with OS X 10.9 Mavericks but was upgraded to OS X 10.10 Yosemite. I've recently upgraded to macOS Sierra and am experiencing some strange behaviour on startup where the screen has turned bright purple until I get to the login screen where it goes back to normal. The nMP is connected via HDMI to HDMI to an Asus 27" 1080p external monitor. I have tried PRAM and SMC resets using different cables, ports, monitors and adapters. I have also run the Apple Diagnostics test and it has resulted in 'no issues found'. Does anyone else have this issue or have an idea what it could be? Thanks for reading!