Correct font properties for subtitles
For some reason the default subtitle colour often seems to be yellow; I can't find any definitive reason why though. I hate yellow subtitles with a passion, they always look hideous and distracting, so I always make mine white. To separate them from light coloured backgrounds I use a soft black drop shadow if possible, or otherwise a black border. As you can see below they are readable even over light backgrounds.
The size depends on the viewing medium. The one below looks a bit wee in the size it's displayed on this page (on a retina screen on a 15&quot; laptop), so if I was doing subtitles for handheld devices where it might be displayed at this size I'd make it larger, but on a TV it's plenty big enough. In a cinema it might be a bit too large. Resolution is a factor, too. If this was going to standard definition I'd have to bump up the font size a bit too.
Sans-serif is my default choice, usually Arial because it's so bland, so it doesn't distract from the art direction. Serif fonts are also a problem if the video is ever going to be displayed on interlaced screens (which applies if it is to be broadcast, at least until all the old interlaced TVs have gone to the verge), because the thin horizontal lines flicker between fields. Also at low resolutions the thin parts of the letters get lost. So while serif fonts generally have greater readability they might not be the best choice just yet.