Certain games will show yellow, which means that the game's romset has the files needed, but there's something wrong, like the name of the zip file, or there are uneeded files in the zip, it will tell you what the problem is. If you right click and select repair, it will make the needed changes, and then show green. If a game shows red, it means one or more files is missing from the zip. In that case, you need to find another romset for the game in question. I don't think you'll come accross too many reds, if any.