iMac video projector issue

We use a 2017 iMac running Mojave and our video playback has turned dark and dull when using our old 480dpi Eikii projector. The display software does not seem to affect the output or allow us any control. If I hook up a base Win 10 laptop with only onboard graphics the videos run bright and sharp. I am the volunteer IT person in a small church with a PC background so any help would be much appreciated.