Purple display with green apple logo after trying to wake

I replaced the hard drive in a 2014 iMac today and everything went smoothly and I was able to install the iMac just fine. However, after doing this, if the iMac goes to sleep/the display turns off, it doesn’t turn on the display. The system is running underneath. After some time, the display will turn back on, with a purple display and a green Apple logo (see image below). The iMac than still responds to keystrokes, specifically the keystroke for reboot (ctrl+cmd+disk eject), and it will reboot just fine.

Has anyone here experienced this, and has an idea of how to fix this? I already tried reinstalling the system and the apple hardware test comes out clean (for what it's worth). As far as my knowledge goes, this can't be caused by the display cables. Because the display does work just not after the iMac going to sleep.

EDIT: I tried a couple of things, first of all I tried to see if the backlight had failed, which it has not. Second an external display, this does work after it goes to sleep once, after that it also doesn’t work anymore, and after some time it will reboot on it’s own with the second display.

The display I get