The manual for Moment Of Silence says that used dialog lines should show up darker than the unused lines so that you can tell which questions have already been asked, but I can't see any difference. Used and unused lines both look like the same yellow color. Is there a fix for this? I'm using the 4 CD version put out in the US by The Adventure Company.

Chances are that "feature" just doesn't work for newer versions of Windows than the game was made for. At the time the game came out, Windows XP SP2 was current.

I installed it on my old XP SP2 computer and I still don't see any difference. The used and unused lines still look exactly the same.

I don't remember if the feature worked for me or not.
I played it over 10 years ago in Windows 2000.

I checked the YouTube playthrough ***here*** where he's talking to Bob, and the difference in color is pretty subtle, but visible. Do you see the difference in the color of the lines in the video?

No, I don't see any difference in the lines. They all look the same yellow color to me. I had already watched a video before I made my last post and forgot to mention it. The one I watched was not the same one that you linked to. In yours you could hear comments from the gamer, and the one I found didn't have any audible comments, at least not in the parts I watched. The one I found was made by 123pazu, who has made a ton of video walkthroughs. In both of those videos, as well as both of my computers at home, I can't see any difference in the color of the used and unused lines.