Opacity Discrepancy

My coworker and I have the same settings in the Properties panel, but our text background looks very different. Both of these are at 50% opacity. My coworker's captions always come out darker and I can't figure out why! We're having the same issue with drop shadows - his are always much darker than mine. If I open his projects on my computer, they still show darker than mine. Is there some color or graphics setting that is causing this? Please help! Thank you!